As far as some people’s assurance that Christians somehow cannot backslide- it just is not Scriptural or true experientially. Let us look at the Scriptures:

Remember therefore how you have received and heard; hold fast and repent. Therefore if you will not watch, I will come upon you as a thief, and you will not know what hour I will come upon you.
You have a few names even in Sardis who have not defiled their garments; and they shall walk with Me in white, for they are worthy.
He who overcomes shall be clothed in white garments, and I will not blot out his name from the Book of Life; but I will confess his name before My Father and before His angels.

So what does this passage say?

Remember therefore how you have received and heard; hold fast and repent.​

Jesus wanted the people at this church to remember how they had received. Did they receive? Absolutely, Jesus would know better than your pastor. So although they had genuinely received and heard, these Christians were in need of repentance. Anyone who says they had never actually received is arguing against Who? Jesus said they “received” that is the absolute end of discussion for us who know Him as the Son who is absolute in His authority.

He who overcomes shall be clothed in white garments, and I will not blot out his name from the Book of Life; but I will confess his name before My Father and before His angels.

Here Jesus says that the Christians who overcome will not be blotted out from the Book of Life. Is it all who received and heard, or just those who overcome? Your pastor vs Jesus the Christ- you pick who is right. Or who you think is right anyway.

Imagine reading these Texts and actually believing them:

For if you live according to the flesh you will die; but if by the Spirit you put to death the deeds of the body, you will live.

and

You see then that a man is justified by works, and not by faith only.

and

For as the body without the spirit is dead, so faith without works is dead also.

and

For if, after they have escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and overcome, the latter end is worse for them than the beginning.
For it would have been better for them not to have known the way of righteousness, than having known it, to turn from the holy commandment delivered to them.
But it has happened to them according to the true proverb: “A dog returns to his own vomit,” and, “a sow, having washed, to her wallowing in the mire.”