All OT prophesy pointed to the Messiah. The Messiah fulfilled those prophesies. Just two examples would be Isaiah 53 and Psalm 22 which happen to be prophesy foretelling Jesus' death.
Exactly which prophesy do we read about foretelling of any prophet that would arrive after Jesus Christ?
Evidence of Jesus' death? His disciples fled and feared and denied prior to his death. Afterwards, they had absolutely no fear of anything in this life. Preaching Jesus - and what they knew He did for us - was their #1 concern - no matter the cost. Why? They saw Jesus defeat death and sin - Then and only then, they were absolutely certain he was the Messiah.
He didn't come back in the Kingly form the Jews had hoped for and expected and His Kingdom wasn't what they had expected either. But, what it was did fulfill ALL Old Testament prophesy.
And, we're given warnings in the New Testament that even if an Angel teaches something contrary to what they've taught, it is incorrect and we are not to believe it.
Where in the Old Testament can we read about the one you follow, besides in Ishmael?
No Messiah was promised to the Jews by God. There is no prophesy about a messiah by God at all in the Hebrew Scriptures.
Only a prophet was promised in Deut 18:18-19 and after Moses, none of the sort described to Moses, came to Israel. Jesus does not fit the description given in Deut 18. And I believe that Muhammad, our Prophet fits and meets that description.
You quoted from Galatians 1:8 and under that clause, you are not supposed to accept Mark, Matthew, Luke, John and others, who stand accursed in Paul's view.
So, why do you not follow only what Paul preached?