apostasy
The abandonment or renunciation of a profession of the Christian faith.

• From scripture

Now the Spirit expressly says that in later times some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons… (1 Timothy 4:1 ESV).

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).

• From Why Some Leave Christ by Charles Spurgeon:

In all our churches, among the many who enlist, there are some who desert. They continue awhile, and then they go back to the world. The radical reason why they retract is an obvious incongruity. “They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us” (1 Jo 2:19). The unconverted adherents to our fellowship are no loss to the Church when they depart. They are not a real loss, any more than the scattering of the chaff from the threshing-floor is a detriment5 to the wheat. Christ keeps the winnowing fan always going. His own preaching constantly sifted His hearers. Some were blown away because they were chaff. They did not really believe. By the ministry of the Gospel, by the order of Providence, by all the arrangements of divine government, the precious are separated from the vile, the dross is purged away from the silver [so] that the good seed and the pure metal may remain and be preserved. The process is always painful. It causes great searching of heart amongst those who abide faithful and occasions deep anxiety to gentle spirits of tender, sympathetic mold…I put it to myself. I put it to those who are the officers of the church. I put it to every member without exception: Will ye also go away? 

• From An Exposition of Hebrews by Arthur Pink:

 [I]t needs to be remembered that all who had professed to receive the Gospel were not born of God: the parable of the Sower shows that. Intelligence might be informed, conscience searched, natural affections stirred, and yet there be “no root” in them. All is not gold that glitters. There has always been a “mixt multitude” (Ex. 12:38) who accompany the people of God. Moreover, there is in the real Christian the old heart, which is “deceitful above all things and desperately wicked”, and therefore is he in constant need of faithful warning. Such, God has given in every dispensation: Genesis 2:17; Leviticus 26:15, 16; Matthew 3:8; Romans 11:21; 1 Corinthians 10:12.

Finally, let it be said that while Scripture speaks plainly and positively of the perseverance of the saints, yet it is a perseverance of saints, not unregenerate professors. Divine preservation is not only in a safe state, but also in a holy course of disposition and conduct. We are “kept by the power of God through faith”. We are kept by the Spirit working in us a spirit of entire dependency, renouncing our own wisdom and strength. The only place from which we cannot fall is one down in the dust. It is there the Lord brings His own people, weaning them from all confidence in the flesh, and giving them to experience that it is when they are weak they are strong. Such, and such only, are saved and safe forever.