If they depart or recant the faith as you suggest, they were not genuine to begin with. They professed only, but in actually did not “have eternal life.” They tasted, they heard, and knew the message, and experienced the convicting power of the Spirit, yet never truly repented, accepting Jesus Christ as Lord and Savior. That which is “born of God,” the new creature in Christ, cannot and will not ever deny Him.
This is the typical OSAS and Calvinistic means of reconciling the requirement that saving faith must have “things which accompany salvation” to be considered saving faith, while in contrast, many today hold to the specious doctrine that as long as one believes Jesus promise to give eternal life than he/she is saved.
However, the classic approach must construe warnings given to believers against rejecting their faith, and making Christ “of none effect,” and profiting them nothing, (Gal. 5:1-14) and becoming adversaries of God,(Heb. 10:27-31) to being only applicable to unbelievers, but which restriction the texts will not allow, as it was beleivers who were being specifically addressed.
There is also the additional reality that Paul also had assurance that some souls were part of the elect, based upon their tested faith, (1Thes. 1:4ff) yet there are souls even today who have manifested “works meet for repentance,” and a good degree of tested faith and virtue, only to latter die in adulterous relationships or the like.
Thus i find it more warranted that just as a soul makes a decision to believe, being enabled and persuaded by the grace of God, and which results in imputed righteousness, they also can choose to will-fully rebel or otherwise recant in times of testing, and forfeit that which faith procures, or at least they are warned against doing so, which clearly infers they could.
Yet we are also given assurance that by continued growth in grace, we may “make our calling and election sure: for if ye do these things, ye shall never fall.” (2Pet. 1:10) To God be the glory, though i certain do not feel close to having that condition.