Jamieson, Fausset and Brown on 2 Corinthians 13:3

3. Since--The reason why he will not spare: Since ye challenge me to give a "proof" that Christ speaks in me. It would be better if ye would "prove your own selves" (2Co 13:5). This disproves the assertion of some that Scripture nowhere asserts the infallibility of its writers when writing it.

which--"who" (Christ).

is not weak--in relation to you, by me and in this very Epistle, in exercising upon you strong discipline.

mighty in you--has given many proofs of His power in miracles, and even in punishing offenders (2Co 5:11, 20, 21). Ye have no need to put me to the proof in this, as long ago Christ has exhibited great proofs of His power by me among you (2Co 12:12) [Grotius]. It is therefore not me, but Christ, whom ye wrong: it is His patience that ye try in despising my admonitions, and derogating from my authority [Calvin].