Okay.
It is not that Jesus "became" One with the Father, but rather that He became a man after already being One with the Father.
But our perspective and our disadvantage viewpoint that has cause Jesus' identity to appear almost opposite of what is actually true. In reality, "He was, and is, and is to come" is the proper order and perspective. Meaning: He was One with the Father, then is or was the Christ, then again was and now is come One...as ultimately, He always was.
Becoming the Christ was just a side trip to do Himself what no one else could successfully do. Such is the parable of the wicked vinedressers (Matthew 21:33-41).
All of which also means that all was before the foundation of the world, and is only now revealed through the course of time. Which is the grace of God to bring His children through all that was before, in a slow and gentle manner. Anything less would ultimately be too traumatic for children to survive it. So, all of history (His story), is rather "precept upon precept, line upon line, here a little there a little" until all is revealed--which is not God's reality, but rather His story...like the words of a book that is already written and only now read in our hearing. "Then comes the end"--the end of this time of gentle revelation of all that was before the foundation of the world.