The Spirit explicitly says that in later times some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons, by means of the hypocrisy of liars seared in their own conscience as with a branding iron. (1 Timothy 4:1-2).
The sower went out to sow his seed; and as he sowed, some fell… on rocky soil, and as soon as it grew up, it withered away, because it had no moisture… Those on the rocky soil are those who, when they hear, receive the word with joy; and these have no firm root; they believe for a while, and in time of temptation fall away. (Luke 8:5-6,13).
In the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, and then have fallen away, it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God, and put Him to open shame. (Hebrews 6:4-6).
See also Luke 9:62, 2 Thessalonians 2:3.