I always get this question, "What about the thief on the cross? He was not baptized." This is posed as the ultimate gotcha, like I've never considered it before. Actually the retort is very simple: the thief died in the OT under law, not under the NT under Grace.
15 Therefore he is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, since a death has occurred that redeems them from the transgressions committed under the first covenant. 16 For where a will is involved, the death of the one who made it must be established. 17 For a will takes effect only at death, since it is not in force as long as the one who made it is alive.
The word Testament means "Will" like "Last Will and Testament." Many don't understand that the New Testament (Will) didn't begin until Jesus died and the Will read.
To help you understand, let's pretend you have an Aunt Loopy and you are hoping she is going to leave you her vast fortune. Of course, it is necessary before you get anything that she dies first. You won't get anything unless Aunt Loopy trips over one of her cats and breaks a hip. Then after the funeral you must wait until the will is read, because as far as you know, she still could have left everything to Mr. Fluffy.
Good news, Jesus did put you in His will and He didn't leave anything to Mr. Fluffy! But there are conditions to receive your inheritence like in many wills. Frankly you must hear the will read (Rom 10:17), believe what is written (Mark 16:16), repent of your behavior (Acts 17:30), confess your agreement with the will as read (1 Tim 6:12), and be at Jesus' funeral (Rom 6:3-5).
The New Testament did not begin until Jesus died and the gospel preached (the reading of the will) which is recorded in Acts chapter 2. Most don't realize the ministry of Jesus was technically under the Old Testament Law. When Jesus died and raised from the dead, then we have the transition to the New Testament. Peter's sermon in Acts 2:1-36 is essentially the will being read, Acts 2:37-38 are the conditions, and Acts 2:41 is the receiving of the inheritance by following the conditions.
So baptism, a New Testament practice, would not pertain for a thief dying under the OT Law. Jesus was speaking to the thief, which means Jesus was still alive, and second, the Will wasn't read until Peter declared it on Pentecost 50 days later. When Jesus said, "You will be with me in paradise" it was consistent with the Old Testament Law as Jesus was his required sacrifice. Again, most people don't realize that all of the OT saints were saved by the cross, because they were looking forward to it in the same way that we today look back (Heb 9:15).
Known as an expert in all he surveys, he freely shares his opinions on politics, science & theology using diatribes based upon careless research from tertiary sources that presupposed what he thought in the first place.