The source of the "carnal Christian" term is primarily from the King James where Paul writes, "And I, brethren, could not speak unto you as unto spiritual, but as unto carnal, even as unto babes in Christ." (1 Cor 3:1) Of course, this is simply an older English version of today's "flesh". Paul writes to the church at Corinth that they were still suffering from "the flesh", a term referring to the "old self", the worldly ways. He lists things like "jealousy and strife" (1 Cor 3:3). In Galatians he explains,
Now the works of the flesh are evident: sexual immorality, impurity, sensuality, idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God. (Gal 5:19-21)That's "the flesh" -- carnal.
So what do people mean by "carnal Christian"? Well, typically, they mean that the Spirit of God does not dwell in them. They are saved, but they have no outworking of the Spirit ... because they don't have the inworking of the Spirit. He's not there. They are spiritually alive, but with nothing to show for it.
We arrive here primarily by anecdotal evidence. We all know people who claim to be Christians but exhibit no signs of it or people who once seemed to be Christians but have since ceased. Therefore, we all know that the "carnal Christian" -- a saved-but-bereft-of-the-Spirit Christian -- is a real thing. We've seen it. But, just as both science and the courts would prefer not to rely solely on anecdotal evidence, I would urge us not to come to a position based purely on our observation. What does the Bible say?
According to Scripture, at the point of repentance, "you will receive the gift of the Holy Spirit." (Acts 2:38) This is the "indwelling". He comes to take residence in the believer. There is no suggestion that this is a "come and go" condition like it was in the Old Testament (e.g., 1 Sam 16:14). In fact, the Spirit is described for Christians as the "seal" of our inheritance (Eph 1:13). Paul says it is by the Holy Spirit that we "were sealed for the day of redemption." (Eph 4:30) Now, we can grieve the Spirit (Eph 4:30) and resist the Spirit (Acts 7:51) and quench the Spirit (1 Thess 5:19), but Scripture argues that His indwelling is permanent. Scripture speaks of being filled with the Spirit (literally, "be being filled with the Spirit", an ongoing thing) (Eph 5:18), but there is no hint anywhere that one who is born anew can be without the Holy Spirit entirely. He may be more or less under the influence of the Spirit, but not without Him. It isn't the difference between a growing Christian and a not-growing Christian. It is the difference between a genuine Christian and one who is not.
One text where this is unavoidable is in 1 John. There we read,
No one born of God makes a practice of sinning, for God's seed abides in him, and he cannot keep on sinning because he has been born of God. (1 John 3:9)Now, the Bible has many uses of the word "seed" in reference to God's Word or the Gospel. It's in the Parable of the Sower (Matt 13:3-8; Matt 13:19). In Peter's first epistle he refers to an imperishable seed by which we are born again, "the living and abiding word of God" (1 Peter 1:23). But the Word as seed is not the exclusive use for "seed". Paul refers to the believer's body as a seed (1 Cor 15:38) from which the glorified body is derived. As it turns out, scholars are not agreed on just what that "seed" is. But here is what we can clearly and unavoidably see. First, the reference is to those who are "born of God". The object of this verse is those people. Second, the one who is born of God does not make a practice of sinning. Not "might not" or "should not" -- does not. Third, regardless of what God's seed actually is, the verse clearly states that it is outside the power of the one born of God to "keep on sinning". "He cannot."
From Scripture, then, we know that the Holy Spirit is given at the new birth. We know that, unlike the Old Testament, there is no biblical argument that the Holy Spirit ever leaves again. There may be more influence or less, but there will be no absence. And we know that one who is born of God does not make a practice of sinning because he cannot make a practice of sinning. As such, it can only be concluded that the concept of the "carnal Christian" is a mistaken concept. Surely there are individuals who claim to be Christians but are not (1 John 2:18-20). Certainly there are genuine Christians who fall into sin (1 John 2:1), even grievous sin. But the idea of a genuine believer, born again, indwelt by the Holy Spirit, who simply fails to change at all ever -- while passing the anecdotal evidence test -- fails to pass the biblical test. It would suggest we need to reconsider our "carnal Christian" concept and, more to the point, our understanding of those we think are carnal Christians.