We believe in the Christ of history, but those who lived before Jesus believed in the Christ of prophesy. That would take in Abraham, Jacob, Joseph, Moses, David, Solomon, and so on. They believed in the Messiah, a synonym for "Christ." So they were the first "Christians."
No man ever tried to find happiness without God more than Solomon. No man ever strayed farther from God than Solomon. Yet read what God revealed to David about Solomon, his much loved son, in 2 Samuel 7:12-15...
When your days are over and you rest with your fathers, I will raise up your offspring to succeed you, who will come from your own body, and I will establish his kingdom. He is the one who will build a house for my Name, and I will establish the throne of his kingdom forever. I will be his father, and he will be my son. When he does wrong, I will punish him with the rod of men, with floggings inflicted by men. But my love will never be taken away from him, as I took it away from Saul, whom I removed from before you.
This text is why I believe Solomon is a Christian who is with God in heaven today. God loved him with an everlasting love.
It's doubtful that you or I could ever stray further or farther than Solomon. And that's why I believe the Holy Spirit preserved his story--and 2 Samuel 7:12-15--so that any believer can find comfort and encouragement when we lose our bearings for a season. No matter how long or short that season is lasting, the love of God for us is "ever" lasting.