There's no such thing as a carnal Christian, only carnal people who believe in Christ. The demons believe, but are not Christians. A Christian is a follower of Christ and the Spirit, not of the flesh which is carnal (Gal.5:16-6:8; Rom.8:1-13). If a Christian has stepped off the road that is narrow, it is only due to unforgiveness, hatred, unbelief, and/or sin. All of which is a ceasing of following Christ, even for a moment, which means they are no longer a Christian, for the ways of the flesh will not lead anyone through the narrow gate, even if they've been on the narrow path (Lk.13:24-27; 1 Cor.6:9-10; Gal.6:7-8; 1 Pt.4:1-19). One must repent, for this is the only way to start following Christ when past sin is held against you (Eze.18:20-32; Lk.13:3, 5; Jn.3:1-7; Rom.3:25; 6:16-23; 1 Jn.1:9).