RE: What is the origin of the Trinity?
I'm afraid we can't blame the doctrine of the Trinity on the Catholics or on any old myths that happen to have three of something.
Jesus himself made it very clear that He and the Father were one and He referred to himself as the Son of God.
For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. For God did not send his Son into the world to condemn the world, but to save the world through him. Whoever believes in him is not condemned, but whoever does not believe stands condemned already because they have not believed in the name of God's one and only Son. This is the verdict: Light has come into the world, but people loved darkness instead of light because their deeds were evil. Everyone who does evil hates the light, and will not come into the light for fear that their deeds will be exposed.
First of all back up and see it says God "sent" his son. You can "send" yourself you only send others.
Also notice what Jesus said about being "one" woth God.
"11 “I am no longer in the world, but they are in the world, and I am coming to you. Holy Father, watch over them on account of your own name, which you have given me, so that they may be one just as we are one."
Notice Jesus said his desciples would be one in the same manner that he and God are one. Are we all the same person? Jesus surely wasn't saying we were all part of one organism, but he was saying we would be one in thought, and belief. So him and God are one just as we are one.
No. The doctrine of the Trinity says "God in three persons, not the same person" And yes, in the next life we will be children of God and part of that same oneness.
But Jesus said just as we "are" this is present tense. Why is Jesus one with God before ascending to heaven? Apparently Jesus was one with God while human (one in thought, same goals) and we can be too by thinking inline with his word.
Yes! Jesus was simultaneously fully God and fully man. Google "hypostatic union".
That said, there were certain prerogatives of deity he lay down before his resurrection. One of them was certain knowledge of when he would return, for example. Quite a mystery, I'll admit.