He wasn't so much "rejected" as never accepted in the first place. The Torah was given and the Torah is the Ultimate Revelation. Nothing any prophet says may contradict it in any way and any "prophet" who prophecies it ceasing and being replaced (or "fulfilled" as christians dishonestly put it) is false.
Why don't you accept Joseph Smith?
He was not rejected by all, that much is a certainty. There were more than a few "Jews" as it were (as they were?) who did accept him, even among the Sanhedrin.
Then there were the Pharisees who truly did hate him, and all his followers with the hottest of hatreds.
One of them, a young Pharisee of Pharisees named Saul, most likely would have "kept up the good work" of eradicating the followers (killing them, actually) had he not had a supernatural encounter on the road to Damascus.
What is the Torah (and here may I assume you mean Tanakh ---or do you intend books of the law only?) but record of the law which was given (along with some setting, some lead-up from Adam to Abram, and from there to the deliverance of the Hebrews from bondage in Egypt), additional accounting of these chosen one's interaction with God, with Him speaking through prophets to them, along with a few other inspired writings such as Psalms & Proverbs?
I can tell you what it isn't. It is not the additional commentaries which developed alongside Torah.