One of the most comforting Scriptures for me is found in 2 Timothy 2:19: “Nevertheless, God’s solid foundation stands firm, sealed with this inscription: ‘The Lord knows those who are his.’”
Many commentators link this verse to Numbers 16 in the OT. Paul, of course, was an OT scholar, one of the best. He would have remembered the incident Moses had with Korah. Korah, a Levite, along with some Reubenites (Dathan, Abiram and On) rose up against Moses. They convinced 250 other men to join them.
When Moses was confronted, he fell down on his face, and then said to Korah and all his followers: “In the morning the LORD will show who belongs to him and who is holy, and he will have that person come near him.” [vs5]
The LORD definitely showed the Israelites who belonged to Him when the ground split apart and swallowed up Korah and the rebels.
When I pastored there were times when people died that I just did not know about their spiritual condition. I cannot read hearts. Thankfully, God can. He knows if they accepted His salvation with their dying breath. He knows the state of their heart and the state of their minds when they stand before Him. I can trust Him to always do what is right by each of us.
What I would say to all, however, is to not leave anyone in doubt of your salvation. Make sure your family, your friends know that “you are His.” Don’t leave any to worry what your final destination is at your passing!