"How can Christians say that Jesus is the only way to God?"
Jesus Himself claimed to be God and others recognized Him as Deity. He said, "I and the Father are one" (John 10:30). The Greek language, in which the New Testament was originally written, is explicit. Literally, Jesus said, "I and the Father, we are one." Jesus also claimed that He was the only way to God: "I am the way, and the truth, and the life; no one comes to the Father, but through Me" (John 14:6). Again, the Greek language is very clear. Jesus used the words ‘the way' and not ‘a way.'
The question is, can we accept as true what Jesus [God the Son], claimed? The most conclusive evidence establishing the deity of Christ is the historically verifiable resurrection. The apostle Paul wrote that Jesus "was declared with power to be the Son of God by the resurrection from the dead." (Romans 1:4) If the resurrection of Christ is true, then it is possible and, indeed, logical to believe all His claims and miracles including His assertion of the complete validity of the Old Testament. They would be what one would expect from God.
The primary reason that Jesus is the only way to God is that He forgives sin and offers eternal life, (John 10:28) something that the founder of no other major religion has claimed to do. Think about that!
Every person is separated from a perfect, holy God by sin. According to God's requirement, either the individual must pay the penalty for sin — death — or he must find someone who is willing and able — who has no sin — to pay the penalty for him. (Hebrews 9:14, 22) Jesus Christ was both able and willing to do this, as demonstrated by His death on the cross. That is why Jesus is the only way to God.