The simple answer is that they were never in the faith.
They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us.1 John 2:19 (ESV)
But there is more that can be said.
Often times the reason why those in ministry leadership positions ‘depart’ from the faith is that they have fallen in love with something in the world that finally exposed the god they’ve been seeking for, even in Christianity. Demas, who was a fellow missionary with the Apostle Paul, is a Biblical example of what is still very common today (Compare Col 4:14 with 2 Tim 4:10). As one seminary professor warned the class one day, the three F’s of pastoral failure is finances, females, and fame—all three in the top ten gods of this world that the true God warned, and even commanded, us against. Theft, licentiousness, and moving on to ‘greater’ things are often the sins that precede the public dissent from the faith, or that follow shortly after the public treason against Christ.
Another reason why well trained ‘Christians’ often walk away from Christianity is that they have come to love theological criticism more than they have come to love theological Truth. Many lesser-trained Christians are often guilty of the same. We love what might be true more than we love what is revealed to be true. We find doubt more intriguing than assurance; or we love “testing everything” more than we love “holding fast to what is true” (1 Thess 5:21). The Apostles of Jesus warned us against following new ideas instead of old Truth (Col 2:8, 18).
Those who ‘depart’ from the faith had never joined the faith. They might have joined themselves to Christianity and persuaded many for a long time, but their reasons for joined themselves to Christianity were never having been born again (1 Jo 3:9). They might have joined out of conservative tradition handed down by their parents, they might have joined out of spiritual affirmation by their peers, they might have joined out of personal spirituality that warmed their hearts for some time, but the reality is that as obvious as their external association to Christianity was, so obvious their internal resistance to Christ was. It might not have been evident all at once, but eventually even they no longer believed the lie of their testimony of faith.
How do we not fall under the same deception? Simple. Believe the Scriptures which you claim to believe, know the LORD whom you claim to believe, and turn away from the evil that you are attracted to.
Stating the same believe, know, turn instruction in different words,
Be not wise in your own eyes;Proverbs 3:5 (ESV)
fear the LORD, and
turn away from evil.