*Minor differences ignored. Grouped by changes, with first version listed as example.
Not that we are competent.  When he thus disclaims all merit, it is not as if he abased himself in merely pretended modesty, but instead of this, he speaks what he truly thinks. Now we see, that he leaves man nothing. For the smallest part, in a manner, of a good work is thought. In other words,  it has neither the first part of the praise, nor the second; and yet he does not allow us even this. As it is less to think than to will, how foolish a part do those act, who arrogate to themselves a right will, when Paul does not leave them so much as the power of thinking aught!  Papists have been misled by the term sufficiency, that is made use of by the Old Interpreter.  For they think to get off by acknowledging that man is not qualified to form good purposes, while in the mean time they ascribe to him a right apprehension of the mind, which, with some assistance from God, may effect something of itself. Paul, on the other hand, declares that man is in want, not merely of sufficiency of himself, (autarkeian,) but also of competency (hikanoteta,)  which would be equivalent to idoneitas (fitness), if such a term were in use among the Latins. He could not, therefore, more effectually strip man bare of every thing good. 
1 - "Non point que soyons suffisans;" -- "Not that we are sufficient."
2 - "Pour le moins;" -- "At least."
3 - See Institutes, volume 1. -- Ed.
4 - Wiclif (1380) following, as he is wont, the Vulgate, renders the verse as follows: "Not that we ben sufficiente to thenke ony thing of us as of us: but oure sufficience is of God." -- Ed.
5 - "La disposition, preparation, et inclination;" -- "Disposition, preparation, and inclination."
6 - Charnock, in his "Discourse on the Efficient of Regeneration," makes an interesting allusion to Calvin's exposition of this verse. "Thinking," says he, "is the lowest step in the ladder of preparation; tis the first act of the creature in any rational production; yet this the Apostle doth remove from man, as in every part of it his own act, (2-Corinthians 3:5) Not that we are sufficient of ourselves to think any thing as of ourselves, but our sufficiency is of God. The word signifies -- reasoning: no rational act can be done without reasoning; this is not purely our own. We have no sufficiency of ourselves, as of ourselves, originally and radically of ourselves, as if we were the author of that sufficiency, either naturally or meritoriously. And Calvin observes, that the word is not autarkeia, but hikanote" -- not a self ability, but an aptitude or fitness to any gracious thought. How can we oblige him by any act, since, in every part of it, it is from him, not from ourselves? For as thinking is the first requisite, so it is perpetually requisite to the progress of any rational act, so that every thought in any act, and the whole progress, wherein there must be a whole flood of thoughts, is from the sufficiency of God." -- Charnock's Works, volume 2, p. 149. -- Ed.
Not that we are sufficient of ourselves - This is evidently designed to guard against the appearance of boasting, or of self-confidence. He had spoken of his confidence; of his triumph; of his success; of his undoubted evidence that God had sent him. He here says, that he did not mean to be understood as affirming that any of his success came from himself, or that he was able by his own strength to accomplish the great things which had been effected by his ministry. He well knew that he had no such self-sufficiency; and he would not insinuate, in the slightest manner, that he believed himself to be invested with any such power, compare note on John 15:5.
To think anything - (λογίσασθαι τι logisasthai ti). The word used here means properly to reason, think, consider; and then to reckon, count to, or impute to anyone. It is the word which is commonly rendered impute; see it explained more fully in the note on Romans 4:5. Robinson (Lexicon) renders it in this place, "to reason out, to think out, to find out by thinking." Doddridge renders it, "to reckon upon anything as from ourselves." Whitby renders it, "to reason; as if the apostle had said, We are unable by any reasoning of our own to bring people to conversion. Macknight gives a similar sense. Locke renders it, "Not as if I were sufficient of myself, to reckon upon anything as from myself:" and explains it to mean that Paul was not sufficient of himself by any strength of natural parts to attain the knowledge of the gospel truths which he preached. The word may be rendered here, to reckon, reason, think, etc.; but it should be confined to the immediate subject under consideration. It does not refer to thinking in general; or to the power of thought on any, and on all subjects - however true it may be in itself but to the preaching the gospel. And the expression may be regarded as referring to the following points, which are immediately under discussion:
(1) Paul did not feel that he was sufficient of himself to have reasoned or thought out the truths of the gospel. They were communicated by God.
(2) he had no power by reasoning to convince or convert sinners. That was all of God.
(3) he had no right to reckon on success by any strength of his own. All success was to be traced to God. It is, however, also true, that all our powers of thinking and reasoning are from God; and that we have no ability to think clearly, to reason calmly, closely, and correctly, unless he shall preside over our minds and give us clearness of thought. How easy is it for God to disarrange all our faculties, and produce insanity! How easy to suffer our minds to become unsettled, bewildered, and distracted with a multiplicity of thoughts! How easy to cause every thing to appear cloudy, and dark, and misty! How easy to affect our bodies with weakness, langor, disease, and through them to destroy all power of close and consecutive thought! No one who considers on how many things the power of close thinking depends, can doubt that all our sufficiency in this is from God; and that we owe to him every clear idea on the subjects of common life, and on scientific subjects, no less certainly than we do in the truths of religion, compare the case of Bezaleel and Aholiab in common arts, Exodus 31:1-6, and Job 32:8.
Not that we are sufficient of ourselves - We do not arrogate to ourselves any power to enlighten the mind or change the heart, we are only instruments in the hand of God. Nor was it possible for us apostles to think, to invent, such a scheme of salvation as is the Gospel; and if we even had been equal to the invention, how could we have fulfilled such promises as this scheme of salvation abounds with? God alone could fulfill these promises, and he fulfils only those which he makes himself. All these promises have been amen-ratified and fulfilled to you who have believed on Christ Jesus according to our preaching; therefore, ye are God's workmanship and it is only by God's sufficiency that we have been able to do any thing. This I believe to be the apostle's meaning in this place, and that he speaks here merely of the Gospel scheme, and the inability of human wisdom to invent it; and the words λογισασθαι τι, which we translate to think any thing, signify, properly, to find any thing out by reasoning; and as the Gospel scheme of salvation is the subject in hand, to that subject the words are to be referred and limited. The words, however, contain also a general truth; we can neither think, act, nor be, without God. From him we have received all our powers, whether of body or of mind, and without him we can do nothing. But we may abuse both our power of thinking and acting; for the power to think, and the power to act, are widely different from the act of thinking, and the act of doing. God gives us the power or capacity to think and act, but he neither thinks nor acts for us. It is on this ground that we may abuse our powers, and think evil, and act wickedly; and it is on this ground that we are accountable for our thoughts, words, and deeds.
Not that we are sufficient of ourselves to think any thing as of ourselves; but our (e) sufficiency [is] of God;
(e) In that we are proper and able to make other men partakers of so great a grace.
Not that we are sufficient of ourselves,.... Though we are sufficient for this work to which God has called us, and have such trust and confidence that he has blessed and owned us, and done such great things by us; yet we do not ascribe anything to ourselves, to any power of ours, to any self-sufficiency in us: for "we are not sufficient of ourselves" neither for the work of the ministry, nor for the conversion of sinners, nor for faith and hope in God, nor for any spiritual work whatever; not even to think anything as of ourselves; any good thing, either for our own use and benefit, or for the advantage of others; we are not able of ourselves to meditate with judgment and affection upon the word of God, to study the Scriptures, to collect from them things fit for the ministry; and much less with freedom and boldness to speak of them to edification; and still less able to impress them upon the heart: for though you who are the epistle of Christ are ministered by us, yet not by any power and self-sufficiency of ours;
but our sufficiency is of God; to think, to speak, and to act for his glory.
The Greek is, "Not that we are (even yet after so long experience as ministers) sufficient to think anything OF ourselves as (coming) FROM ourselves; but our sufficiency is (derived) FROM God." "From" more definitely refers to the source out of which a thing comes; "of" is more general.
to think--Greek, to "reason out" or "devise"; to attain to sound preaching by our reasonings [THEODORET]. The "we" refers here to ministers (2-Peter 1:21).
anything--even the least. We cannot expect too little from man, or too much from God.
Not that we are sufficient of ourselves - So much as to think one good thought; much less, to convert sinners.
*More commentary available at chapter level.