On a number of questions about Muslims' dealings with Christians or Christianity, someone will say something like, "Not all Christians believe Jesus is God or the Son of God." While that statement is technically true, it's important to get a handle on just how many Christians believe it, and how incredibly unlikely it is that you'll ever meet a Christian who doesn't believe it.
According to Pew (as of 2011), there are about 2,184,060,000 Christians in the world. Of those, 2,155,630,000 (98.7%) identify as either Catholic, Protestant, or Orthodox. All three of those groups explicitly believe the doctrine of the trinity, which means (among other things) they believe Jesus is God.
Of the 1.3% (28,430,000) of Christians who are "other," it's mostly Mormons (who believe Jesus is literally God's son) and Jehovah's Witnesses and Christadelphians (who believe Jesus is divine, and an incarnation of the angel Michael). There are also modalists, who believe in the deity of the Father, Son, and Holy Spirit, but deny that they are three persons. Then there are unitarians, who are eclectic in their beliefs, but are united by their disbelief in the deity of Jesus -- however, most unitarians are not exactly practicing Christians and may even identify as atheists (it's similar with Quakers). Then you've got Armstrongists, who believe God is a "family," and that Jesus is God and so are we. A good overview of nontrinitarians can be found on Wikipedia. As you can see, even among the very few Christians who don't believe the trinity, there are no notable sects that don't believe Jesus is either God, the Son of God, or "divine" in some way.
You may be able to find individual people who self-identify as Christians while believing Jesus was merely a man, but such people will generally identify as members of one of the groups I mentioned -- just liberal ones. It's analogous to people who identify as Muslims without believing that the Qur'an was actually dictated by Gabriel to Muhammad.
The situation may have been slightly different in Muhammad's time, but not much. The Ethiopian king his followers sought shelter from would have been Ethiopian Orthodox, a subgroup of the "Orthodox" mentioned above. Sources are inconsistent about what sect Bahira the monk and Khadija's cousin Waraqa ibn Nawfal belonged to, but they mention these possibilities:
Mandaeanism, which does not identify as Christian since they believe Jesus was a false Messiah and false prophet
Nestorianism, which affirms Jesus' deity, though it has an idiosyncratic view of how his deity and humanity coexist in his person
Arianism, which denied Jesus was God but did say that he "subsisted before time" and is "as perfect as God"
Syriac Orthodox, another subgroup of "Orthodox"
So even in Muhammad's time there don't seem to have been a significant number of Christians denying Jesus' identity as God or God's Son or in some way divine.
These are just things you should be aware of if you're writing a question or answer about Islamic-Christian relations. If you would like to make a correction or ask for clarification or evidence, please do so below.
If you see someone claiming that not all Christians believe Jesus is God or the Son of God, it may be useful to point them here.