I want to be outrageous:
To tell the difference between Islam and Christianity in a nutshell. In two words, literally: the two key words that name them.
What do they mean?
Islam means submission: the image it evokes is that of a sovereign, under whom stand all people as his subjects.
Christ means anointed: the image it evokes requires a few words more of explanation.
In the Old Testament the kings of Israel were anointed with perfumed oil, which made them brilliant and smelling of good. Anointment, chrisma, was then the sign of election by the only sovereign to represent him among the people.
To call Jesus Christos was to proclaim him king.
Now, his followers are called Christians, but they are actually themselves christoi: at baptism, in fact, and again at confirmation, they receive the chrisma.
The image then is not of people commanded to submit themselves to the sovereign, but called by him to share of his sovereign kingship.