First of all, the Bible promises that you can know for sure, and because the Bible is reliable and God’s Word then the promises can be trusted. Plus, the Bible tells us that God has placed His Spirit in those who believe in Him.
So His Spirit cannot go anywhere but to be in the presence of God for eternity.
Also, that one aspect of salvation is that you have been adopted into God’s family and even here on earth once you are adopted you can never be disinherited according to man’s laws how much more would God’s laws be eternal?
Some lessons have : What is the role of the Holy Spirit in the Christian’s life?
The Holy Spirit lives in us and convicts of us sin and gives guidance. A future lesson has more details.
Attempt to share the Gospel to draw them to Christ and/or ask, “Would you like to have a one on oneBible study to see some answers for yourself?”