The thief on the cross could not possibly believe that because Jesus had not died, let alone been raised. Hence, people who believe faith is essential to salvation cannot use the thief as their example for how to be saved. He was no more saved by the faith of the gospel than he was saved by baptism. We could as easily use the thief to prove salvation without Scriptural faith as we could to use him as an example of salvation without baptism. Even those who believe in salvation by faith only must admit we today are not saved like the thief was saved.
Many Scriptures clearly teach that baptism is essential to receive forgiveness under the gospel.
He who believes and is baptized will be saved. God's word does not contradict itself. It clearly teaches baptism is essential to receive remission. Therefore, the thief cannot prove people ARE saved without baptism for that would contradict other Scriptures. So there must be some other explanation for the case of the thief.