Since ye seek a proof of Christ speaking in me, which to you-ward is not weak, but is mighty in you.
Since - the reason why he will not spare: since ye challenge me to a "proof" that Christ speaks in me. It would be better if ye would "prove your own selves" (2 Corinthians 13:5). This disproves the assertion that Scripture nowhere asserts the infallibility of its writers when writing it. Christ spake through Paul; lived in Paul (Galatians 2:20); suffered in Paul (Colossians 1:24; Acts 9:4-5).
Which - `who' (Christ).
Is not weak - not without power of chastisement in relation to you, by me generally, and in this very letter.
Mighty in you - has given proofs of power in miracles, and in punishing offenders (1 Corinthians 5:5). Ye need not put me to the proof. Long ago Christ has exhibited His power by me among you (2 Corinthians 12:12). It is not me, but Christ, whom ye wrong, by derogating from my authority.