Some thoughts... (none of them terribly original)
1. We are saved by grace through faith
Eph 2:8-10
8 For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— 9 not by works, so that no one can boast. 10 For we are God’s handiwork, created in Christ Jesus to do good works, which God prepared in advance for us to do.
In this passage Paul tells us...
We are saved by grace through faith
NOT BY works
We are saved by grace through faith
TO DO good works
There is no work we can perform to gain salvation.
There is no work we can perform to lose salvation.
2. Those who believe are saved
John 3:18
18 Whoever believes in him is not condemned, but whoever does not believe stands condemned already because they have not believed in the name of God’s one and only Son.
Jesus is pretty clear on this point
Those who believe are saved.
Those who do not believe are not saved.
3. Which brings us to the main question of this thread.
Can a person who has believed in Jesus change their mind and choose not to believe in Jesus?
I actually think Scripture gives us a clear answer to this question
1 John 2:19
19 They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us.
According to John
If a person truly puts their faith in Jesus, then they will remain with Jesus.
If a person leaves Jesus, then that is evidence that they did not truly belong to Jesus in the first place.
We can't always tell the difference between wheat and tares/weeds.
That is the point Jesus makes in Matthew 13:24-30.
Those who genuinely put their faith and trust in Jesus are wheat, and will remain with Jesus.
If a person apostasizes and rejects Jesus, then according to John that is evidence that they are really a "tare/weed" and never really belonged to Jesus.