I here a lot of assumptions be made here and doctrine of men. The Bible clearly states what baptism is for ( Acts 2:38, Mark 16:15-16, Acts 22:16, 1 Peter 3:21, Acts 10:48, John 3:5, Luke 7:30) just to name a few. The thief on the cross, we do not have enough info on, you can only make an assumption as which you did. Plus the thief was under the old law when he died, also Matthew 9:2-7 shows us Jesus could forgive sins. Just to keep in mind in Ephesians 2:8 Paul is speaking to Christians. Nowhere will you find baptism being some kind of show of faith. The Bible is pretty clear on what it is. You are teaching doctrine of man.
