I was having a conversation with a friend who believes that people can lose their salvation. I believe in once saved, always saved. She used Saul as an example of someone who was saved, then lost their salvation. I know I still believe in OSAS, but how do I explain this. Does 1 Samuel 28:19 prove that Saul is saved, when Samuel says tomorrow he will be with him “Moreover the LORD will also deliver Israel with thee into the hand of the Philistines: and tomorrow shalt thou and thy sons be with me: the LORD also shall deliver the host of Israel into the hand of the Philistines.”
The problem with your friend’s Saul argument is that it doesn’t take into account the unique relationship the Church has with the Holy Spirit. Nowhere in the Old Testament is the Holy Spirit described as being sealed within a person, but it’s a promise to everyone in the Church.
After sinning with Bathsheba, David pleaded with the Lord not to take the Holy Spirit from him (Psalm 51:11). You and I don’t pray that prayer because God can’t take his Holy Spirit from us. He’s sealed inside us as a deposit guaranteeing our inheritance.
There are so many theological problems with the notion of a person becoming “unsaved” that it would be rather like the physical problems you would encounter in trying to become “unborn.”
It implies that the Lord doesn’t really know those who are His like He claims to, that you don’t have the authority to become a child of God, that He can’t keep those the Father gives Him as He promises, and that the Holy Spirit can be “unsealed” from within us, meaning He wasn’t really a guarantee of our inheritance after all.
It means that although salvation is a free gift given to whoever asks regardless of merit, the Lord will take it back if He later discovers that we didn’t deserve it.
Of course that means that the Lord can change His mind about us, implying He doesn’t really know the end from the beginning, that we’re not really a new creation in violation of Paul’s argument in 2nd Corinthians 5:17, that there is condemnation for those who are in Christ Jesus in violation of his argument in Romans 8:1, and that something can separate us from the love of God in violation of his promise in Romans 8:38-39.
It means that after beginning with the Spirit, we must continue with human effort in violation of Galatians 3:3, and that He Who began a good work in us can’t really carry it on to completion, a violation of Philippians 1:6.
I could cite dozens of other verses but I think you get the point. For further study, I’ve written several articles on the subject. (You can see the search for this term on the site here: Search GTF for Eternal Security)
As for 1 Samuel 28:19, it’s a prophecy of Saul’s impending death. The “me” in the passage refers to Samuel, not the Lord. Samuel had died earlier and Saul was speaking with him through the witch of Endor. Samuel is saying that like him, Saul would also be in the spirit world the next day.