*Minor differences ignored. Grouped by changes, with first version listed as example.
If we confess He again promises to the faithful that God will be propitious to them, provided they acknowledge themselves to be sinners. It is of great moment to be fully persuaded, that when we have sinned, there is a reconciliation with God ready and prepared for us: we shall otherwise carry always a hell within us. Few, indeed, consider how miserable and wretched is a doubting conscience; but the truth is, that hell reigns where there is no peace with God. The more, then, it becomes us to receive with the whole heart this promise which offers free pardon to all who confess their sins. Moreover, this is founded even on the justice of God, because God who promises is true and just. For they who think that he is called just, because he justifies us freely, reason, as I think, with too much refinement, because justice or righteousness here depends on fidelity, and both are annexed to the promise. For God might have been just, were he to deal with us with all the rigor of justice; but as he has bound himself to us by his word, he would not have himself deemed just, except he forgives.  But this confession, as it is made to God, must be in sincerity; and the heart cannot speak to God without newness of life. It then includes true repentance. God, indeed, forgives freely, but in such a way, that the facility of mercy does not become an enticement to sin. And to cleanse us The verb, to cleanse, seems to be taken in another sense than before; for he had said, that we are cleansed by the blood of Christ, because through him sins are not imputed; but now, having spoken of pardon, he also adds, that God cleanses us from iniquity: so that this second clause is different from the preceding. Thus he initiates that a twofold fruit comes to us from confession, -- that God being reconciled by the sacrifice of Christ, forgives us, -- and that he renews and reforms us. Were any one to object and say, that as long as we sojourn in the world, we are never cleansed from all unrighteousness, with regard to our reformation: this is indeed true; but John does not refer to what God now performs in us. He is faithful, he says, to cleanse us, not today or tomorrow; for as long as we are surrounded with flesh, we ought to be in a continual state of progress; but what he has once begun, he goes on daily to do, until he at length completes it. So Paul says, that we are chosen, that we may appear without blame before God, (Colossians 1:22;) and in another place he says, that the Church is cleansed, that it might be without spot or wrinkle. (Ephesians 5:27.) If yet any one prefers another explanation, that he says the same thing twice over, I shall not object. 
1 - "Faithful" and "just" are nearly of the same import, having both a regard to God's promise, only the latter affords a stronger or an additional ground of confidence, inasmuch as the fulfillment of God's gracious promise is set forth as an act of justice. So that the penitent has here two of God's attributes, faithfulness and justice, to encourage and support his faith. We may, at the same time, consider "just" as having reference to forgiveness, and "faithful" to cleansing, according to a very common mode of stating things both in the Old and New Testament, the order in the second clause being reversed. Then "just" means the same as when Paul says, "that he might be just and the justifier of him that believeth in Jesus," Romans 3:26. Forgiveness is thus an act of justice, then, not to us, but to Christ, who made an atonement for sins. -- Ed.
2 - That is, that he refers to forgiveness in the two clauses. -- Ed
If we confess our sins - Pardon in the Scriptures, always supposes that there is confession, and there is no promise that it will be imparted unless a full acknowledgment has been made. Compare Ps. 51; Psalm 32:1-11;; Luke 15:18 ff; Luke 7:41 ff; Proverbs 28:13.
He is faithful - To his promises. He will do what he has assured us he will do in remitting them.
And just to forgive us our sins - The word "just" here cannot be used in a strict and proper sense, since the forgiveness of sins is never an act of justice, but is an act of mercy. If it were an act of justice it could be demanded or enforced, and that is the same as to say that it is not forgiveness, for in that case there could have been no sin to be pardoned. But the word "just" is often used in a larger sense, as denoting upright, equitable, acting properly in the circumstances of the case, etc. Compare the notes at Matthew 1:19. Here the word may be used in one of the following senses:
(1) Either as referring to his general excellence of character, or his disposition to do what is proper; that is, he is one who will act in every way as becomes God; or,
(2) that he will be just in the sense that he will be true to his promises; or that, since he has promised to pardon sinners, he will be found faithfully to adhere to those engagements; or perhaps,
(3) that he will be just to his Son in the covenant of redemption, since, now that an atonement has been made by him, and a way has been opened through his sufferings by which God can consistently pardon, and with a view and an understanding that he might and would pardon, it would be an act of injustice to him if he did not pardon those who believe on him.
Viewed in either aspect, we may have the fullest assurance that God is ready to pardon us if we exercise true repentance and faith. No one can come to God without finding him ready to do all that is appropriate for a God to do in pardoning transgressors; no one who will not, in fact, receive forgiveness if he repents, and believes, and makes confession; no one who will not find that God is just to his Son in the covenant of redemption, in pardoning and saving all who put their trust in the merits of his sacrifice.
And to cleanse us from all unrighteousness - By forgiving all that is past, treating us as if we were righteous, and ultimately by removing all the stains of guilt from the soul.
If we confess our sins - If, from a deep sense of our guilt, impurity, and helplessness, we humble ourselves before God, acknowledging our iniquity, his holiness, and our own utter helplessness, and implore mercy for his sake who has died for us; he is faithful, because to such he has promised mercy, Psalm 32:5; Proverbs 28:13; and just, for Christ has died for us, and thus made an atonement to the Divine justice; so that God can now be just, and yet the justifier of him who believeth in Jesus.
And to cleanse us from all unrighteousness - Not only to forgive the sin, but to purify the heart.
1. Sin exists in the soul after two modes or forms:
(1.) In guilt, which requires forgiveness or pardon.
(2.) In pollution, which requires cleansing.
2. Guilt, to be forgiven, must be confessed; and pollution, to be cleansed, must be also confessed. In order to find mercy, a man must know and feel himself to be a sinner, that he may fervently apply to God for pardon; in order to get a clean heart, a man must know and feel its depravity, acknowledge and deplore it before God, in order to be fully sanctified.
3. Few are pardoned, because they do not feel and confess their sins; and few are sanctified or cleansed from all sin, because they do not feel and confess their own sore, and the plague of their hearts.
4. As the blood of Jesus Christ, the merit of his passion and death, applied by faith, purges the conscience from all dead works, so the same cleanses the heart from all unrighteousness.
5. As all unrighteousness is sin, so he that is cleansed from all unrighteousness is cleansed from all sin. To attempt to evade this, and plead for the continuance of sin in the heart through life, is ungrateful, wicked, and even blasphemous; for as he who says he has not sinned, 1-John 1:10, makes God a liar, who has declared the contrary through every part of his revelation; so he that says the blood of Christ either cannot or will not cleanse us from all sin in this life, gives also the lie to his Maker, who has declared the contrary, and thus shows that the word - the doctrine of God is not in him.
Reader, it is the birthright of every child of God to be cleansed from all sin, to keep himself unspotted from the world, and so to live as never more to offend his Maker. All things are possible to him that believeth; because all things are possible to the infinitely meritorious blood and energetic Spirit of the Lord Jesus. See the notes on the parallel passages in the margin; and particularly in St. John's gospel, John 1 note.
(6) If we confess our sins, he is (g) faithful and just to (h) forgive us [our] sins, and to cleanse us from all unrighteousness.
(6) Therefore the beginning of salvation is to acknowledge our wickedness and to require pardon from him, who freely forgives all sins, because he has promised to do so and he is faithful and just.
(g) So then our salvation depends on the free promise of God, who because he is faithful and just, will perform that which he hath promised.
(h) Where then are our merits? for this is our true happiness.
If we confess our sins,.... Not to one other; for though it is our duty to confess our faults to our fellow creatures and fellow Christians which are committed against them, yet are under no obligation to confess such as are more immediately against God, and which lie between him and ourselves; or at least it is sufficient to confess and acknowledge in general what sinful creatures we are, without entering into particulars; for confession of sin is to be made to God, against whom it is committed, and who only can pardon: and a man that truly confesses his sin is one that the Spirit of God has convinced of it, and has shown him its exceeding sinfulness, and filled him with a godly sorrow for it, and given him repentance unto salvation, that needeth not to be repented of; and who, under such a sight and sense of sin, and concern for it, comes and acknowledges it before the Lord, humbly imploring, for Christ's sake, his pardoning grace and mercy; and such obtain it:
he is faithful and just to forgive us our sins: forgiveness of sin here intends not the act of forgiveness, as in God, proceeding upon the bloodshed and sacrifice of Christ, which is done at once, and includes all sin, past, present, and to come; but an application of pardoning grace to a poor sensible sinner, humbled under a sense of sin, and confessing it before the Lord; and confession of sin is not the cause or condition of pardon, nor of the manifestation of it, but is descriptive of the person, and points him out, to whom God will and does make known his forgiving love; for to whomsoever he grants repentance, he gives the remission of sin; in doing of which he is faithful to his word of promise; such as in Proverbs 28:13; "and just"; in being "true", as the Arabic version adds, to his word; and showing a proper regard to the blood and sacrifice of his Son; for his blood being shed, and hereby satisfaction made to the law and justice of God, it is a righteous thing in him to justify from sin, and forgive the sinner for whom Christ has shed his blood, and not impute it to him, or punish him for it; though the word here used may answer to the Hebrew word which sometimes carries in it the notion and idea of mercy and beneficence; hence mercy to the poor is sometimes expressed by righteousness; and the righteous acts of God intend his mercies and benefits unto men; see Daniel 4:27; and so forgiveness of sin springs from the tender mercies of our God, and is both an act of justice and of mercy; of justice, with respect to the blood of Christ, and of pure grace and mercy to the pardoned sinner: the following clause,
and to cleanse us, from all unrighteousness, is but the same thing expressed in different words; for all unrighteousness is sin, and to cleanse from sin is to remove the guilt of it, by an application of the blood of Christ for pardon. The antecedent to the relative "he" in the text, is either God, who is light, and with whom the saints have fellowship; or his Son Jesus Christ, who is the nearest antecedent, and who, being truly God, has a power to forgive sin.
confess--with the lips, speaking from a contrite heart; involving also confession to our fellow men of offenses committed against them.
faithful--to His own promises; "true" to His word.
just--Not merely the mercy, but the justice or righteousness of God is set forth in the redemption of the penitent believer in Christ. God's promises of mercy, to which He is faithful, are in accordance with His justice.
to--Greek, "in order that." His forgiving us our sins and cleansing us, &c., is in furtherance of the ends of His eternal faithfulness and justice.
forgive--remitting the guilt.
cleanse--purify from all filthiness, so that henceforth we more and more become free from the presence of sin through the Spirit of sanctification (compare Hebrews 9:14; and above, see on 1-John 1:7).
unrighteousness--offensive to Him who "is just" or righteous; called "sin," 1-John 1:7, because "sin is the transgression of the law," and the law is the expression of God's righteousness, so that sin is unrighteousness.
But if with a penitent and believing heart, we confess our sins, he is faithful - Because he had promised this blessing, by the unanimous voice of all his prophets. Just - Surely then he will punish: no; for this very reason he will pardon. This may seem strange; but upon the evangelical principle of atonement and redemption, it is undoubtedly true; because, when the debt is paid, or the purchase made, it is the part of equity to cancel the bond, and consign over the purchased possession. Both to forgive us our sins - To take away all the guilt of them. And to cleanse us from all unrighteousness - To purify our souls from every kind and every degree of it.
*More commentary available at chapter level.