The thief who was on the right side of the Lord Jesus Christ entered the Kingdom of Heaven without Baptism. How do you justify that? Doesn't that prove that it is by one's faith and not necessarily by Baptism that people are saved?
I would like first to point out that no one has entered the Kingdom of Heaven yet. The thief on the right of the Lord Jesus Christ has been admitted into Paradise, which is different from the Kingdom of Heaven.
The fact that baptism is necessary for our salvation is not a human invention but it is a biblical and divine teaching. Lord Jesus Christ answered, "Verily, verily, I say unto thee, except a man be born of water and of the Spirit, he cannot enter into the Kingdom of God" (John 3:5).
We should not generalize and say that no one will enter Heaven without physical baptism, because this would mean that all the righteous people and all the prophets who lived before the death of the Lord Jesus Christ on the Holy Cross, who had not been baptized, will not enter the Kingdom!
In baptism we die with the Lord Jesus Christ and are born again into a new life "Do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life" (Rom 6:3-4).
This thief was not physically baptized by water but he was washed (baptized) with the Blood of our Lord who died in front of him on the Holy Cross. Besides, the thief had no chance to be baptized. This was an exceptional case, and we cannot take exceptions and make them a general rule. For more information about this question, you may read "Salvation in the Orthodox Concept" by His Holiness Pope Shenouda III and "Comparative Theology" by His Holiness Pope Shenouda III.