Christianity and Judaism are likely the most similar. Christianity and Judaism both believe in one God who is almighty, omniscient, omnipresent, eternal, and infinite.
The all-important difference between Christianity and Judaism is the Person of Jesus Christ. Christianity teaches that Jesus Christ is the fulfillment of the Old Testament prophecies of a coming Messiah / Savior (Isaiah 7:14; 9:6-7; Micah 5:2). Judaism often recognizes Jesus as a good teacher, and perhaps even a prophet of God. Judaism does not believe that Jesus was the Messiah. Taking it a step further, Christianity teaches that Jesus was God in the flesh (John 1:1,14; Hebrews 1:8). Christianity teaches that God became a human being in the Person of Jesus Christ so He could lay down His life to pay the price for our sins (Romans 5:8; 2 Corinthians 5:21). Judaism strongly denies that Jesus was God or that such a sacrifice was necessary.
Jesus Christ is the all-important distinction between Christianity and Judaism. The Person and work of Jesus Christ is the one primary issue that Christianity and Judaism cannot agree upon. In Matthew 15:24, Jesus declared, "I was sent only to the lost sheep of Israel.” The religious leaders of Israel in Jesus' time asked Him, “Are you the Christ, the Son of the Blessed One?” Jesus replied, 'I am,' ... And you will see the Son of Man sitting at the right hand of the Mighty One and coming on the clouds of heaven" (Mark 14:61-62). But they didn’t believe His words or accept Him as the Messiah.