Except that God does many things for, to, and in the genuine believer so that he or she is never going to stop believing in Christ and His finished work of redemption. So this is merely a theoretical fantasy cooked up by those who have never truly believed. The gift of the Holy Spirit, the indwelling Holy Spirit, and the New Birth themselves guarantee that faith in Christ remains firm and continues for all time. And this is not even the half of it.
Any Christian is able to fall away from that faith in Jesus. You may observe this event, failing faith, happening around the world at present.
Christians that are strongly committed to Jesus Christ, are a very small percentage of the Christian world. Christians that devote themselves to the scripture, to prayer, to fellowship. Are highly unlikely to undergo a faith crises.
No one denies that the Holy Spirit is very active in the Christian life.
Unfortunately, the vast majority of churches are in decline.
You need to address this reality.
To say that they all never believed in Jesus is utter fantasy.