Jamieson, Fausset and Brown on Galatians 5:25

25. in ... in--rather, as Greek, "If we live (see on Ga 5:24) BY the Spirit, let us also walk (Ga 5:16; 6:16) BY the Spirit." Let our life in practice correspond to the ideal inner principle of our spiritual life, namely, our standing by faith as dead to, and severed from, sin, and the condemnation of the law. "Life by (or 'in') the Spirit" is not an occasional influence of the Spirit, but an abiding state, wherein we are continually alive, though sometimes sleeping and inactive.