why is it necessary to believe or reject the Trinity
Well for one thing, without divinity, Jesus would have been a liar when he said that he would resurrect himself from the dead. I face plenty of liars in my line of work; don't need another one in my life.
In John 2:19 Jesus predicts that He will raise Himself from the dead (see also John 10:18)
A man who was known even by his enemies as a worker of miracles, said that he woud die as a substitute (scape goat) for the sins of the world. Then, after three days in a tomb, he would resurrect himself from the dead. Only God could do such a thing.
Then, he did it. This is how Christianity got started. And because of this utterly stupendous event, it then spread like wildfire, eventually conquoring the greatest empire the world had ever known, without ever lifting a single weapon.
"And without controversy great is the mystery of godliness: God was manifested in the flesh, Justified in the Spirit, Seen by angels, Preached among the Gentiles, Believed on in the world, Received up in glory". - 1 Tim. 3: 16 KJV
If Jesus isn't "God manifest in the flesh" he could not have resurrected himself from the dead and the whole thing is a hoax, and Jesus is a liar and the worst of the worst charlatans.