Why did Jesus’ disciples change his teachings and start Christianity?

Assalamu alaikum, brothers and sisters. I grew up Catholic in Alabama, but I’ve always been curious about Islam ever since my Catholic high school held interfaith talks with our bishop and a local imam. I’ve been reading some older discussions about Jesus, and it seems like many of you believe he was actually Muslim. I’m puzzled because I learned he was Jewish and clearly taught he was part of the Holy Trinity-Father, Son, and Holy Spirit. Writings from the followers of his disciples, plus first-century Roman historians like Josephus and Tacitus, say the same, and his followers believed that too. How does Islam explain this differently?