Are Christianity and Judaism the same religion or two different religions? Both Jews and Christians recognize that we have different religions; in spite of the fact that Christianity has its origins in the context of 1st century Judaism. Early on in Muslim history the Christian response to Islam was that Islam wasn't a different religion, but a heretical offshoot of Christianity, and was largely treated as such.
That gives some fluidity to the concept here; but nobody today thinks Islam and Christianity are the same religion just of different kinds; likewise the same with Christianity and Judaism.
Sometimes, because of shared ideas, religions have commonalities, as in the case of the Abrahamic religions. But simply because both Christians and Muslims recognize Jesus as having importance doesn't mean there are intensely significance differences.
If we try and find something of a highly minimalistic, kernel of an idea for Christianity, it would look something like this:
Jesus is the Christ, the Son of God of God, our Lord; who suffered under Pilate, was crucified, buried, dead, and rose from the dead. God sent Jesus to save us.
Even without getting into conversations about Christ's Deity, or what it means to say Jesus is the Son of God, or any of those things we have a kernel idea:
-Jesus is the Messiah.
-He is, in a unique way, called the Son of God.
-He is to be called Lord, having magisterial and kingly authority from God in a larger, even cosmic sense.
-He was tried before the Roman governor of Judea, Pontius Pilate, who had Him put to death--and He died.
-After He died, He stopped being dead, being raised up from the dead.
-Jesus is our Savior, He brings salvation.
This kernel of what makes Christianity Christianity is not identical to what makes Islam Islam.
The core, or kernal, the defining feature(s) of Islam are:
-The absolute oneness of God.
-God sends messengers to mankind to return human beings to the true worship of the one God.
-Muhammad is the chief messenger of God, the Seal of the Prophets.
-The Qur'an is the full, final, and perfect revelation from God of what God desires of human beings.
Jesus has a role here, as a messenger of God. But the fundamentally unique quality which Christians ascribe to Jesus is not found here:
When Christians say Jesus is the Messiah, we are speaking of Him as more than messenger or prophet, but as King, even the King of kings. There can be no loftier person than Jesus here. Because He is also called Lord. He is also Savior, the redeemer of the world.
Christianity insists that Jesus is King and Lord.
Christianity insists that Jesus died by crucifixion and was resurrected.
Christianity insists that Jesus is the Savior of the world.
This makes Jesus, in Christianity, not merely an important player; but the chief article of our religion, the object of our devotion and religion. Jesus is absolutely at the center of our faith and practice, our tradition, our worship, our confession--everything about Christianity ultimately is centered upon the figure of Jesus.
Attempting to argue that Christianity and Islam are "basically the same" because we both refer to the figure of Jesus, and use some of the same words: Messiah, born of a virgin, etc; doesn't change the fact that the very core ideas of our two religions are markedly different; and that beyond superficial similarities we are saying radically different things about the figure of Jesus.
In Christianity Jesus is the center of our devotion.
In Islam Jesus is a messenger among many messengers, a very important messenger, but still another messenger among many messengers.
I don't think we can simply pretend that these sorts of differences aren't of such significance as to warrant the recognition of the two as distinct religions with radically distinct foci. It isn't the same thing as the distinction between Catholicism and Protestantism (especially since there is no such thing as "Protestantism", there are many Protestant groups); because at the very core the various branches of the Christian religion are still the Christian religion: the basic core, that the figure of Jesus is the center of our devotional and religious life, is there.
-CryptoLutheran