g Konn,
Consider 1Pet 3:21, where we are told that Baptism is now saving you. Look at the scriptures before this one 18-20, especially verse 20 which tells us that 8 people were saved through the flood of Noah's day. Being on the Ark saved those people, and baptism is likened to that, because it can save today. Notice that there was only One Ark!!
Today there is ONE Baptism, meaning every person must be Baptized in the One Faith, Eph 4:3-6. This one Faith mentioned, is the One Faith that God blesses today, just like there was only one Ark.
Just because you have been baptized in the one Faith does not mean that you are forever saved no matter what you do. The Bible tells us we must endure until the end, meaning until the end of your life or the end of this system of things, or Age.
I think you mentioned John's Baptism. This was only for the Jews. The reason for this baptism was; the Jews were in a Covenant relationship with God, and they had abandoned Him. To get back into His Grace they had to be baptized for forgiveness of the sins against God, for breaking the Covenant. Then the Jews had to be Baptized again, like the Gentiles, in the name of the Father, Son and Holy Spirit.
Just because a person is baptized, does not mean that he is a true Christian. You must be baptized in the one Faith that God approves and then you must live a life of a true Christian for all your life.
All true Christians must follow in the steps of their master, Jesus, 1Pet 2:21. Jesus was baptized, Matt 3:16,17.
Just as the Bible tells us that we must search for the truth, we must search for that one true religion that God blesses, and it is not the one that you would normally think, in fact it is the opposite, 1Cor 1:26-29, Luke 6:22,23,26, John 15:18-20, Matt 10:22, 24:9.