It might be that not all musicians believe in God. But they all believe in Bach.
-- Maurizio Kagel
Bach marks both the highest point and the end of his era, the era of baroque music which is generally understood to have ended with his death. He's notable for having composed pieces in every genre known at the time, with the sole exception that he didn't write operas. Today, one-quarter of a millennium after his death in 1750, he is the most popular composer of "classical" music.
If you don't know Bach yet, I recommend the following: Go to a live performance of St. John's Passion by J.S. Bach. Do it now. Sit through it all the way, and it will change your life. It's not easy to swallow as it's some of the most emotional and complex music ever written, but it's sure worth the time. Then, continue with the immensely joyful Brandenburg Concertos. If you listen to those first and then try to delve into the more mysterious works like the Passions, you might be deceived and/or negatively overwhelmed.