In those days, John the Baptist came preaching in the wilderness of Judea, 2) and saying "Repent, for the Kingdom of Heaven is at hand!" 3) For this is He who was spoken of by the prophet Isaiah, saying "The voice of one crying in the wilderness. Prepare the way of the Lord; Make His paths straight". (Matthew 3:1-3)
Please note that John's message was not about a religion but the Kingdom of Heaven. It is important to understand that John was the most unique prophet of the Bible. In fact, Jesus stated that John was the greatest of all the prophets that had ever lived.
Assuredly, I say to you, among those born of woman, there has not risen one greater than John the Baptist; but he who is least in the Kingdom of Heaven is greater than he. (Matthew 11:11)
Now why does John hold such a prominent position among the prophets? Because all the prophets before John spoke only of the coming of the Messiah King and the coming of the Kingdom, while John had the privilege of announcing, presenting, meeting and baptizing the King of the Kingdom.