I believe that Paul is writing about people who were never truly believers to start with. As Barnes writes in his commentary: "It does not mean that, as individual, they would have been true Christians; but that there would be a departure from the great doctrines which constitute the Christian faith." John Gill writes: "that is, from the doctrine of faith, notwithstanding it is indisputably the great mystery of godliness, as it is called in the latter part of the preceding chapter; for from the true grace of faith there can be no final and total apostasy, such as is here designed; for that can never be lost. It is of an incorruptible nature, and therefore more precious than gold that perishes; Christ is the author and finisher of it; his prevalent mediation is concerned for it; it is a gift of special grace, and is without repentance; it springs from electing grace, and is secured by it; and between that and salvation there is an inseparable connection; it may indeed decline, be very low, and lie dormant, as to its acts and exercise, but not be lost: there is a temporary faith, and a persuasion of truth, or a mere assent to it, which may be departed from, but not that faith which works by love: here it intends a profession of faith, which being made, should be dropped by some; or rather the doctrine of faith."