The Calvinist teaches that once one is saved, they are always saved. In the book, The Five Points of Calvinism by Steele and Thomas, they write, “The elect are not only redeemed by Christ and renewed by the Spirit: they are also kept in faith by the almighty power of God. All those who are spiritually united to Christ through regeneration are eternally secure in Him. Nothing can separate them from the eternal and unchangeable love of God. They have been predestined unto eternal glory and are therefore assured of heaven.”\
Several passages in the Bible teach that a child of God can indeed fall away. Probably the most well-known is Hebrews 3:12-13. The entire book of Hebrews is written for Christians who were in danger of falling away. Hebrews 6:4-6 speaks of the impossibility to renew one once they have fallen away. Other New Testament writers also teach the possibility of apostasy (1 Corinthians 9:24-27; 2 Peter 2:20-21).
God is sovereign and by grace, He extends salvation to all men. God has chosen the plan of salvation through Christ, but God allows man to be a free moral agent, and whether we are saved or lost will be determined by our response. Paul exhorted that we can be “holy and unblameable and unreproveable in his sight” but only “if ye continue in the faith grounded and settled” (Colossians 1:23).