Surely Islam disagrees on perhaps the biggest facet of the Christian faith, that in being that Jesus died on the cross for our sins, a sacrificial lamb for the atonement to allow us to be reconciled with God because ultimately, there is nothing we can do for our own salvation, except faith, faith in Christ in being who he is, the son of God.
If Christ was just a prophet, then the Christian faith crumbles and we don't have forgiveness?
As far as the east is from the west, so far He removed our transgressions from us (Psalm 103:12)
When God washes us, we are clean. Wash me, and I shall be whiter than snow (Psalm. 51:7).
God pardons abundantly (Isa. 55:7), all your iniquities (Ps. 103:3).
And their sins and their lawless deeds I will remember no more (Hebrews 10:17; Jeremiah 31:34).
God says in the Bible, "I, even I, am the one who wipes out your transgressions for My own sake; and I will not remember your sins" (Isaiah 43:25 Heb. 8:12; 10:17).
Of course religions disagree between each others beliefs and doctrines, that is the reasons why there are so many religions in the world. Christian denominations alone are around 33,000 the reasons for so many Christian denominations are mainly based on disagreements over the interpretation of Scripture.
Disagreements over the meaning and purpose of baptism. Is baptism a requirement for salvation or is it symbolic of the salvation process?
Personal issues and power hungry individuals. Joyce Meyer did not like her church, so she start her own, King Henry VIII did not get a divorce from the Pope, so the whole country is no longer Catholic but Anglican. If I dont like it I make my own. Julius August decided to add 2 months to the calendar; July and August, can you really say with no doubt when Jesus was born?
Disagreements over the interpretation of Scripture are taken personally, literally, symbolic or metaphysical. Some examples of this are TV-evangelists, a new fashion this days is every Dick, Tom and Harry decided to publish his own Bible explaining and adding to it; Joyce Meyers Bible, Joel & Victoria Osteen Bible, Benny Hinn Bible, etc.
Disagreements over the Trinity. Not every Christian denomination beliefs in the Trinity as the Trinity did not come into place until the First Council of Nicaea which was 325 years after Jesus, therefore for the first 325 year AD and for the rest of the BC there was not Trinity, just the first command, that there is only One God not 3 in one.
Disagreements over Jesus being God. As well as above, some Christians belief Jesus was human, a man, a teacher, who discover the divinity of man, his own divinity.
So regarding your sermon, here a few things I have learn from talking and debating with Muslims:
Whenever they talk about God and Mohammed they always say Blessed be their Name this show me that they do not take the name of God in vain.
I was very impress when I asked about Jesus and not only they belief in Jesus as well (although as a holy prophet) but they also say Blesses be his name every time they mention Jesus name. So far, I have not come across a Christian, which shows this kind of respect for the name of Jesus.
6 "I said, 'You are gods; you are all sons of the Most High.' Psalm 82:6
34Jesus answered them, "Is it not written in your Law, 'I have said you are gods'[a]? John 10:34
Jesus: "I tell you the truth, if you have faith and do not doubt, not only can you do what was done to the fig tree, but also you can say to this mountain, 'Go, throw yourself into the sea,' and it will be done. Mathews 21:22