Christianity is defined as believing in Jesus Christ or the fact that he was the son of God 2000 years ago and died on the cross as a sacrifice to save us from original sin.
Judaism is a monotheistic religion that believes in the same God that Christians do only they deny the teachings of Jesus Christ.
Muslim/Islam are belivers in a second prophet who walked the earh around 1000 AD named Muhammed and the writings of the Korran. Again, their God is the same God that the Jews and Christians worship, just different lessens generated from different prophets.
I am a christian because I follow the ways of Jesus Christ. I am a Catholic because I worship in a traditional Catholic perspective of Christianity which it is definitely a part of. There is no separation. Baptists, Protestants, Evangelicals, etc. are all more religions created in the name of Christianity. Regardless of how we worship God, we all believe in the existance of Jesus Christ the son of God who sacrificed his life for us.