Jamieson, Fausset & Brown

Literally, "That we may have no advantage gained over us by Satan," namely, by letting one of our members be lost to us through despair, we ourselves furnishing Satan with the weapon, by our repulsive harshness to one now penitent. The loss of a single sinner is a common loss; therefore, in Co2 2:10, he said, "for your sakes." Paul had "delivered" the offender "to Satan for the destruction of the flesh, that the Spirit might be saved" (Co1 5:5). Satan sought to destroy the spirit also: to let him do so, would be to give him an advantage, and let him overreach us. not ignorant of his devices--"Ignorant" and "devices" are words akin in sound and root in Greek: we are not without knowledge of his knowing schemes.