It shall turn to you for a testimony - That is, it shall turn out on your part for a testimony to them (your persecutors) that you are thoroughly persuaded of the truth of what you teach, and that you are no impostors.

And it shall turn to you for (c) a testimony.
(c) This will be the result of your troubles and afflictions: they will be witnesses both before God and man of the treacherous and cruel dealing of your enemies, as well as of your steadfastness: A noble saying, that the afflictions of the godly and holy men pertain to the witness of the truth.

And it shall turn to you for a testimony. By this means they would have an opportunity of leaving their testimony for Christ before kings and rulers; and what they should meet with from them, would be a means of strengthening and confirming them in the truths of the Gospel; and be a proof and evidence to them of the certainty of the above things Christ had said should be accomplished; as well as be for a testimony against the rulers and governors, Jews, and Gentiles, before whom they should be convened; see Matthew 10:18.

for a testimony--an opportunity of bearing testimony.

It shall turn to you for a testimony - Of your having delivered your own souls, and of their being without excuse.