Proponents of once saved, forever saved are people who teach that once you accept Jesus as your personal Lord and Saviour, it doesn’t matter what you do after. But that’s not what the Bible teaches.
In 1 Timothy 4:1, we are told that, “Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils”. In fact, we are also warned about deliberately sinning after receiving the knowledge of Jesus Christ. “For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins”, (Hebrews 10:26). See Hebrews 2:1-3 also. Peter also told us that it would be worse for people who accept Jesus Christ and later deny Him than those who didn’t believe in the first place, (2 Peter 2:20-22).
So you could see that a person who is saved by accepting Jesus Christ as his/her Lord can lose his/her salvation if they later deny Jesus (either through actions or words) as the only source of salvation. In other words, the person accepted Christ but no longer believes. If such a person who denies Jesus dies today, he/she will miss heaven.
It is for this reason, we are warned in Hebrews 3:12 to “Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.” And we’ve been encouraged to restore people who fall away, (Hebrews 6:6, Galatians 6:1).
In conclusion, be very careful of latter day preachers of “grace”, who hide under the umbrella of grace to tell you to do whatever you like, because it doesn’t matter. They are lying to you – that is not what the Bible teaches. Don’t listen to them. Follow what the Bible teaches on godly living.