If a person as a child believed in Jesus Christ? He will be saved.
If he grows older? And learns to be habitually arrogant? Without God's grace he can not know what he needs to about God. He will never know he was saved until after he finds himself in heaven, stunned to be there.
"But He gives a greater grace. Therefore it says, “God is opposed
to the proud, but gives grace to the humble.” James 4:6
That was written to the church. To believers! Arrogant believers are devoid of God's enabling power of grace. It even says that God "makes war" against them. They can not possibly know they are saved as long as they do not humble themselves. Its not a formula to learn. Its a heart attitude one directs towards God.
There are people, who when they grew up and compared themselves to other people... perceived themselves as innately superior to others in a way they should not. Because, of that arrogance? God opposes them and they are denied the grace power to even know they are saved. I am not saying that its applicable to anyone here. But there are people who are saved and feel nothing towards God. It might apply.
Jesus had to make it clear that he would lose none of all who believed in him. Made it clear for a good reason. For, He knew that not everyone who comes to him would end up looking like a good Christian.
What do I speak of?
"All those the Father gives me will come to me, and whoever comes to me
I will never drive away. For I have come down from heaven not to do my
will but to do the will of him who sent me. And this is the will of him who
sent me, that I shall lose none of all those he has given me, but raise
them up at the last day." John 6:37-39
None will become lost. King Saul became jealous and wanted to murder David. He could no longer know he was saved. Its one reason why he sought out the witch of Endor was to try to gain some supernatural guidance. God stopped making Himself knowable to king Saul because Saul became blatantly arrogant. Many others will be more subtle.
Just before he was to die, the prophet Samuel told king Saul that after he died his death of punishment from God, that Saul would end up being with Samuel... Samuel was without a doubt, saved.