Mohammed came along with new ideas. So did Joseph Smith, and the chick who founded the 7th Day Adventists, and Jim Jones, and Benny Hinn, and Marcion, and whoever started Catharism, etc, etc, etc. Which new ideas do you jump on as the new revelation just like Jesus brought, and which do you reject?
Good point. Note that at the time of Jesus, the orthodox and the most learned rejected Jesus. Those who followed him were not scholars, but simple folk, fisherman, common men and women. For the establishment religious leaders, their knowledge was an obstacle to understanding Jesus.
Rev. Moon says that John the Baptist was the biggest obstacle to Jesus. John was a popular figure at the time with thousands of followers. His father was a leader in the temple.
Although he testified to Jesus at the River Jordan, he did not unite with Jesus and while in prison sent followers to ask Jesus if he was the promised one. Jesus was John's cousin and he must have heard the rumors about Jesus. John was a big shot and Jesus, at the time, was a nobody. He should have been Jesus' 1st disciple but he went his own way, got entangled in some stupid affair and lost his life.
So the question... how to know when a new idea is genuinely from God is a good one. Orthodoxy, knowledge, position and power were obstacles at the time of Jesus. The ones who saw Jesus for what he was were humble and sincere. Humility, sincerity and prayer are the only way to sort things out.