Didn't even see the question the first time, but just so happened to stumble onto it now.
A few reasons I believe what I do
1. When Jesus rebukes the Church of Smyrna in the Book of Revelation, he says nevertheless, I have this against you, that you left your first love. Left is past tense of leave and leaven is sin. Also, could be a reference to the Sons of God shouting with joy when the world was created. How did that happen BEFORE the first begotten Son of God Jesus was crucified? All things are happening simultaneously which is why Jesus said "I Come Quickly" 2k years ago. It's also why he mentions "some standing here won't have the time to witness in all the cities of Israel". Einstein said it's true and Jesus's response when asked: "You're not 50 and you've seen Abraham?" To which Jesus said, "Verily, Before Abraham was, I AM" A constant state of present tense.
2. Kamal's testimony. This man followed Allah and came to the U.S. to wage jihad on Christians and Jews. He broke his neck in a car accident, was cared for by Christian men and taken care of which confused him. He cried out to Allah with no response, then decided he would commit suicide because he doubted his faith. Jesus showed up in his room as Kamal was about to pull the trigger. He says, "I have known you before I formed the foundation of the world, I have loved you BEFORE I formed you in your mother's womb...rise up Kamal. You are my warrior, not theirs!"
"The voice knew me by name--Why don't you call on the God of Abraham, Isaac, and Jacob"
3. Then there's John the Baptist who Jesus said was the greatest born of a woman. Nevertheless, he who is LEAST is the kingdom of heaven is greater than he. The two thirds of the angels that never left home are greater than the ones on this planet who need salvation and redemption.
4. Adam was created, then Eve was taken "out" of Adam and created from his rib to be his help mate. So 1 became 2. The Bible talks about a mystery where the 2 will become one when married--Is that each person's other half?
5. When the disciples ask Jesus about marriage during the resurrection, Jesus rebukes them saying there is NO MARRIAGE in the resurrection, all shall be like the angels of heaven. To me, that sounds like the 2 become one again or something like that. That also alludes to the fact that the ones who left were angels at one time, likely the 1/3 who followed Lucifer to this world--Selling your soul for wealth, fame, power, etc is definitely outlined in scripture and Jesus gave the example response when Satan tempted him with all power over future kingdoms, thou shalt worship the Lord thy God and Him only.
There are more reasons, but I'm going to sleep now...very tired.
Steve M