It has been said that of all the religions in the world, Christianity is the only one that depends on facts for faith. "If the facts aren't true, the faith is foolish."
There is more primary and secondary source evidence for the fact of the birth, life, death and resurrection of Jesus Christ, than there exists in relation to the existence of Julius Caesar. The whole world acknowledges the person of Jesus in its use of the dating system.
The most important fact that the Christian Faith is based on is the resurrection of Jesus from the dead. By the resurrection God attested that everything Jesus said of Himself, and of Humanity's need for a Saviour, is true. Jesus said, "No man comes to the Father but through Me."
Faith is not complicated. Faith is simply acknowledging the fact that I am a sinner in need of a Saviour; it is believing that Jesus died for my sin; it is thankfully surrendering my life to the Living Lord, Who in fact rose from the dead, and now lives.
As we approach Christmas time, we prepare to celebrate the fact of Christ's birth. His life has become real to me -- a fact of life -- which is why, "I believe ..."