John the Baptist's purpose was to point the way to Christ. He did baptize for remission of sins, but the New Covenant was still not in effect until Christ's ultimate sacrifice for our sins. My point is that the argument using the "Thief on the Cross" to show that baptism is not required for salvation is a baseless argument.
I'm understanding that John the Baptist was not included in the era of the Kingdom of God. He was the voice of one crying in the wilderness, "PREPARE the way of the Lord..." That way was not part of John's mission. In fact, the baptism of John was INEFFECTIVE for the Kingdom of God and a Christian era baptism was required of disciples of John in the book of Acts.
Mt 11:12 And from the days of John the Baptist until now the kingdom of heaven suffereth violence , and the violent take it by force.
It was a "baptism of REPENTANCE for the remission of sins." The baptism was a sign of the preceding repentance which was for the remission of sins. That is why obedience to this instruction gives an indication of true belief EXCEPT in cases of extremity such as the thief on the cross.