What is the most significant theological difference between Muslims and Christians

Both religions are monotheistic and accept that the God that appeared to Abraham is the true God.

I think the best way to summarize their theological differences would be by their main Revelation from God, which is different for both.

For the Christians, the Word of God is the Holy Bible.

For the Muslims, the Word of God is the Holy Qur'an.

Both religions follow their respective Holy Books.

Additionally, while the Muslims believe Jesus to be a prophet, Christians believe him to be the Son of God himself.