I wish my Christian brethren would give up on this "thief on the cross" argument. It is a horrible argument to make for these reasons:
1. Yes salvation is by faith in Jesus. But not that Jesus is or even that he is God. It is by faith in a very specific thing.
2. That thing is the gospel message (I Cor 15:1-4)
3. The gospel is that "Jesus died for our sins, was buried, and rose again"
4. Faith/Salvation in the church age/Christian era requires believe in those 3 things (death for my sins, burial, resurrection).
5. The thief on the cross did not believe any of those 3 parts of the gospel as none of them had even happened yet.
6. Yes the thief on the cross was saved by faith but the faith of the Old Testament Theology/era/dispensation. There was no baptism as part of the Old Testament (Jewish) salvation/theology. The bible says "Abraham believed God and it was counted to him as righteousness" (note - this was before his circumcision - Paul makes a huge point about this in the whole chapter of Romans 4).
7. Baptism is part of the New Testament/Christian/Church age theology and in EVERY single case in the bible was administered after faith/belief in the gospel. Also in every case in the Bible it was done immediately after belief (not weeks, months, etc. later - it is a travesty to make people go through a 13 week membership class before they can be baptised).
The thief on the cross has nothing to do with the baptism arguments/studies.
Absolutely.
Yes, by grace alone through faith alone on account of Christ alone. We agree on this. But God always ordained means by which He provides His grace. This means is baptism. I agree that every believer would want to be baptized. I agree that you can be a believer before you are baptized. But you have to convince yourself that a baby is not born in sin to say that baptism does not save him.