One of the oddest things about record of early Islam is the assertion that Waraqah, supposedly a Nazarene (what Arabs call Christians), recognized that Muhammad was a prophet, and yet Muhammad and the Quran address practically no Christian beliefs or practices accurately, even when they try to correct them.
The Name of Jesus
One obvious issue is the name of Jesus. Muhammad calls him Isa, or Esau, a name used as slur used by Jews against Yeshua or Yesu’, as Christian Arabs and Jews called Jesus. All the Islamic sources get Jesus’ name wrong every single time.
Muhammad thinks that Maryam (Mary) the mother of Jesus is the sister of Harun (Aaron) and the daughter of Imran (Amran). That Mary (or Miriam–same name) lived about 1400 years before Mary the mother of Jesus was born, and she was the sister of Moses. When a group of Christians said that Aaron lived many hundreds of years before Mary the mother of Jesus, Muhammad replied that lots of people had the same name back then, and then he later over-corrected wildly by saying that the sister of Moses was named Kulthum, instead, apparently not aware that the Christians were saying he mixed up two different Marys, not that he was giving Aaron’s sister the wrong name. Muhammad also thought that Mary never got married–Joseph was entirely unknown to him.
The Doctrine of the Trinity
The doctrine of the Trinity is three persons on one God. God the Father, God the Son, and God the Holy Spirit are all persons of the one God Yahweh. They are distinct but inseparable. God Yahweh is what the persons are–their separate names are who they are. This is possible because God is perfectly transcendent, all-powerful, and completely other. Muhammad did not even understand that this is what the Trinity taught. He thought the Trinity referred to a triad of gods, consisting of Allah (God proper), Jesus, and Mary. The Quran has multiple statements decrying this belief that literally no one has ever had.
Muhammad is not even aware that baptism exists. If he’d had any contact with any Christians early on, there would have been some reference to baptism, the most important of Christian sacraments, in the Quran or the ahadith.
If Waraqah had been a Christian, he would have made sure that Muhammad was baptized if he actually though Muhammad was a prophet. Then again, he would have told Muhammad Jesus’s name….
Eucharist, or the Lord’s Supper
Likewise, there is zero awareness that Christians have the practice of taking communion/the eucharist/the Lord’s Supper. This would have been a part of every single Sunday service back then and is considered the most important of the sacraments.
Chrism, or Anointing with Oil
Muhammad has no idea that Christians anoint with oil–not those entering the priesthood, the sick, or the dying. This may have been a bit more obscure, but anyone who knew anything at all about Christians would know the first two.
Muhammad had no idea that Christians sang hymns and psalms in their worship services. At this point, singing would have probably been still purely vocal, unaccompanied by instruments, but it would have already been far more musical than the Judaic practice, which was tied to tonal chanting that is much more similar to Islamic Quran recitations.