If a born again Christian backslides and doesn’t confess and repent of his sin, he will not be counted among the faithful. The secret of continued salvation is in continued abiding in Christ. If a person does not abide in Christ, he withers and dies. When the branch is separated from the vine, the source of life is gone. Even a believing, trusting Christian who is connected to the living vine may choose to separate from the vine. So, he can choose to be lost no matter how saved he once was.
Here are some Bible references:
“I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing. If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them and cast them into the fire, and they are burned.” John 15:1-6.
“If that which ye have heard from the beginning shall remain in you, ye also shall continue in the Son, and in the Father.” 1 John 2:24.
“If any man draw back, my soul shall have no pleasure in him.” Hebrews 10:38.
“If a man abide not in me, he is cast forth as a branch.” John 15:6.
“If a man keep my saying, he shall never see death.” John 8:51.
“If thou continue in his goodness: otherwise thou also shalt be cut off.” Romans 11:22.
“If ye do these things, ye shall never fall.” 2 Peter 1:10.
“If we endure, we shall also reign with him: If we deny him, he also will deny us.” 2 Timothy 2:12 (RSV).
“If we sin willfully . . . there remaineth no more sacrifice for sins.” Hebrews 10:26.
From the above verses, we learn that as long as the believer chooses to connect himself to the Lord, his salvation will be secure in God. But if the believer chooses to disconnect himself from God, then, he will be overcome by the devil.
For more on this topic: https://bibleask.org/can-a-believer-lose-their-salvation/
In His service,