First, baptism didn't start with Christians. It was the common Jewish practice that John the Baptist was promoting on the banks of the Jordan River. The Pharisees that came to challenge him never asked, "What are you doing by dunking these people in the water?" They asked, "Why are you doing it?"
John the Baptist wasn't an Essene, as some Bible teachers have suggested. Essenes would undergo ritual baptism (in Hebrew, "mik-vah") at least once a day, and often more. Part of the Essene doctrine was that every transgression of sin required its own mikvah, so they were a pretty saturated group of people. If John were an Essene, the Pharisees wouldn't have needed to ask anything. They would have known why he was baptizing. But, John wasn't an Essene, he was the son of a Levite (a priest) well trained in the traditions and laws of orthodoxy.
For the Jewish people, the tradition of mikvah is personal declaration of renewal. It was necessary to immerse in a flowing pool of water before entering the Temple to worship and anytime one has been to death, sickness, or uncleanness. In the Jewish context, the ritual bath means being rendered clean as serviceable to God.
From a Hebraic mindset, this ritual points directly back to Israel's deliverance from Egypt: passing through the Red Sea.
Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea; And were all baptized unto Moses in the cloud and in the sea...
Baptism is a representation of new birth. It stems from Jewish teachings on the book of Job. The question is asked, "How can that which is unholy become holy?" The answer is that it cannot: it must be made anew. But, going into the waters of mikvah/baptism and coming out the same as we were before does nothing for us. We must understand that we leave something in the water. We need to look back into the depths, just for a moment, to see our enemy being washed away.
Salvation is about being rescued out of bondage, that the wickedness of this world no longer holds us in its grasp. Baptism is about victory. Understanding these in the context of the Exodus story reminds us that the two are not the same thing. This is why so many believers, today, continue to live in guilt and fear.