A proof of Christ speaking in me (δοκιμὴν τοῦ ἐν ἐμοὶ λαλοῦντος Χριστοῦ)
Lit., of the Christ that speaks in me. An experimental proof of what kind of a being the Christ who speaks in me is.
In you (ἐν ὑμῖν)
Better, among you. He is speaking, not of Christ as He dwells in them, but as He works with reference to them (εἰς) and among their number, inflicting punishment for their sin.
Lit., out of, marking the source of both death and life.
Are weak in Him
The parallel with 2Co 13:3 must be carefully noted. Christ will prove Himself not weak, but mighty among you. He was crucified out of weakness, but He is mighty out of the power of God. A similar weakness and power will appear in our case. We are weak in Him, in virtue of our fellowship with Him. Like Him we endure the contradiction of sinners, and suffer from the violence of men: in fellowship with His risen life we shall be partakers of the power of God which raised Him from the dead, and shall exhibit this life of power toward you in judging and punishing you.