Not by Christ, as the formula of an oath, Christ being never used by the apostles in such a formula, but God. Rom 1:9; 2Co 1:23; 2Co 11:31; Phi 1:8. For this favorite expression of Paul, see Gal 2:17; 1Co 1:2; 2Co 2:14, 2Co 2:17; 2Co 12:19, etc.
Conscience
See on 1Pe 3:16.
Bearing me witness
Rev., bearing witness with me. See on Rom 8:16. Concurring with my testimony. Morison remarks that Paul speaks of conscience as if it were something distinct from himself, and he cites Adam Smith's phrase, “the man within the breast.”
In the Holy Ghost
So Rev. The concurrent testimony of his declaration and of conscience was “the echo of the voice of God's Holy Spirit” (Morison).