The necessity of baptism is taught By Jesus and the Apostles..
We see the connection of Baptism to faith in the words of Jesus
"He who believes and has been baptised shall be saved; but he who has disbelieved shall be condemned" Mk16:16
It was Saul who was blinded by the Light on the Damascus Road On that road was where he experienced the reality of Christ risen from the dead and it was, that he received the directive to the stret called straight where he was baptised for the remission of his sins Acts22:16 and thereafter he was commissioned as Apostle Paul.
The baptism of John (the baptizer) was not the same baptism which the disciples were commissioned to perform by the Lord Jesus..
That baptism (John's) was no longer valid after the Death and resurrection of Jesus..
Example: Apollos was teaching John's baptism to the Jews at Ephesus. Upon hearing this, Aquila and Priscilla "expounded unto him the way of God more perfectly" Acts18:25-26
When Paul came to Ephesus he found 12 men that needed to be baptised "in the Name of Christ "having previously received John's baptism Acts 19:3-5...
John's baptism could not cause the believer to be 'UNITED TOGETHER' in the likeness of Jesus' death and resurrection Rom6:5, for Jesus had not yet made the way for this "oneness" in the likeness of His death and resurrection by the provision of His cleansing blood with the remission of sins..
Let us understand: Baptism is much more than just a symbolism of the believers faith in the death and resurrection of Jesus...It is the Blood of Jesus that washes away sin and purges the conscience Heb 9:14
This is the providence of God for sinners.. Baptism is the way which God ordained for the effectiveness of 'the cleansing Blood to be applied to the believer'.. (what Faith recognises, Baptism realises) in other words (Christ shed his blood for all men and it is my Faith in conjunction with my baptism that makes it real for me.. It is my Baptism that spiritually 'unites me to the Cross of Christ' where His Blood was shed for the remission of My sins) Hence Apostle Peter on the day of pentecost said to the repentants who were pierced to the heart, "Let each of you be baptised for the remission of your sins; and you shall receive the Gift of the Holy Spirit" Acts 2:38.
That pattern is followed right Through the Acts of the Apostles in association with the laying on of Hands..
Salvation is not effective to the believer until such Time as the remission of his/her sins has taken place.. Apostle Peter certainly believed that! and he preached it on the day of pentecost.. That is also why Apostle Peter can boldly say using Noah's watery flood in analogy "Baptism Now saves You"
What begins with belief and repentance must follow the ordained precept of God 'Baptism' if one is willing to "fulfil all righteousness" and enter into the Fulness of Life made possible by the new covenant in the Blood of Christ Jesus..