Heb 3:12-14 (emphasis mine)

12Take care, brethren, that there not be in any one of you an evil, unbelieving heart that falls away from the living God. 13But encourage one another day after day, as long as it is still called “Today,” so that none of you will be hardened by the deceitfulness of sin. 14For we have become partakers of Christ, IF we hold fast the beginning of our assurance firm until the end,

1 Peter 3:17
17Therefore, dear friends, since you already know this, be on your guard so that you may not be carried away by the error of lawless men and fall from your secure position.

Why the warnings if we can never fall away? These people they were writing to were Christians due to the reference to "brothers" and "secure position". Did these writers just make a mistake and were concerned about nothing? There are more scriptures about "falling away" and losing Faith, but I don't have time to go into all them.