There are various ways to Biblically test one’s profession of the Christian faith to know who is a real Christian and who is not. A negative test, disqualifying someone, is cited in 1 John 3:15:
Anyone who hates his brother is a murderer, and you know that no murderer has eternal life in him.
Hence, one of the most obvious ways to exclude one from the group possessing salvation is to learn about the sin of murder! It seems almost ridiculous that anyone would think a murderer could have eternal life, but not in our apostate day! If you remember the BTK serial killer, Dennis Rader, and what his Lutheran pastor said about him, then you remember he thought that serial murderer had eternal life, as well as a demon, simultaneously! This is a shocking violation to the true message of the Bible.
The kind of murder John wrote of in 1 John 3:15 is hate. That expands the definition of murder to include hate just as a different Scripture states greed is another form of idolatry (Col. 3:5). So, if a professing Christian is hateful, he is not a real possessor of eternal life at all. We can know that, based on Scripture. That person might have been a Christian at a previous point, but not anymore. As clear as that is, Rev. 21:8 confirms the same by saying a murderer will go to the lake of fire!
But the cowardly, the unbelieving, the vile, the murderers, the sexually immoral, those who practice magic arts, the idolaters and all liars—their place will be in the fiery lake of burning sulfur. This is the second death. (Rev 21:8)
How then can anyone, including the Calvinist, think King David retained his salvation when he turned to both adultery and murder? When King David sinned he lost his salvation, but got it back upon his repentance. Remember, no murderer has eternal life, even though he might have had it previously!