Islamic teachings display a lack of understanding of Christianity. First of all, Muslims use the word “God” as a proper noun (or name), so when they hear Christians say Jesus is God, the Father is God and the Holy Spirit is God, Muslims understand this to mean that the Father is Jesus and is the Holy Spirit (i.e. they refer to the same Person). As for the Trinity, the Qur’an claims that Christians worship three gods (Allah, Jesus, and Mary). As the Qur’an says polytheism can never be forgiven, this is complete anathema for Muslims! No matter how often Christians explain that this is a misrepresentation of our beliefs, Muslims will of course trust what the Qur’an says.
Then of course we have the equation of Christianity with a Western way of life. Muslims outside the West will see the lifestyle shown in Hollywood films and TV series, completely immoral by Islamic standards, and think that this represents Christianity.