The thief on the cross had no baptism.
The thief on the cross died under the Old Covenant. He was never subject to the New Covenant, and so is irrelevant to the discussion of how a NT Christ follower must be saved.
Requiring anything in addition to faith in Jesus Christ for salvation is a works-based salvation.
No, it is not. Rom 10:9-10 clearly places a physical action (confessing Jesus as Lord with the mouth) as a condition upon which the reception of salvation is based.
To add anything to the gospel is to say that Jesus’ death on the cross was not sufficient to purchase our salvation.
Again, no, it is not. Jesus' death is what makes salvation possible in the first place, and without it there would be no hope of Heaven for anyone. God has offered salvation to anyone and everyone, but only those who accept it on His terms will receive it. If you don't publicly and verbally confess Jesus as Lord then He will not confess you as His child at Judgement (you won't be saved) (Rom 10:9-10, Matt 10;32-33). If you do not repent of your sins then God will not forgive you (Acts 3:19). If you are not baptized (in water (1 Pet 3:21)) into Christ's death, then you do not die with Him (Rom 6:1-7), you are not raised with Him (Col 2:11-14), and you are not clothed with Him in righteousness and adopted as a child of God (Gal 3:26-27).
To say that baptism is necessary for salvation is to say we must add our own good works and obedience to Christ’s death in order to make it sufficient for salvation.
Not at all. If I purchase a Rolls Royce for you (pay all the fees you would ever pay and allow refueling for you for the rest of your life), but I tell you that you have to show up at a dealership right down the road from you to pick it up. Does you showing up in any way add to the cost I paid? No! It doesn't at all. But if you don't show up, you don't get to use the car.
Jesus’ death alone paid for our sins seeRomans 5:8; 2 Corinthians 5:21. Jesus’ payment for our sins is appropriated to our “account” by faith alone see John 3:16; Acts 16:31; Ephesians 2:8-9.
What is faith? It is not a mental only process. Faith requires/demands/is dead and useless without action (Heb 11:1, James 2:14-26).
Therefore, baptism is an important step of obedience after salvation but cannot be a requirement for salvation.
Yes, it can. It is during baptism (in water (1 Pet 3:21)) that we die to sin (Rom 6:1-7), that our sin is cut from us by the Holy Spirit (Col 2:11-14), that we are resurrected with Jesus by the power of His blood and the working of the Holy Spirit (Rom 6:1-7, Col 2:11-14), that we are clothed with Christ and adopted as children of God (Gal 3:26-27), that we are washed clean and spotless (Eph 5:26-27), that we enter into the Kingdom of God (John 3:5).
Think about this.... Paul said "I thank God that I baptized none of you except Crispus and Gaius," (And later remembers Stephanus)
If baptism were part of the gospel itself, necessary for salvation, what good would it have done Paul to preach the gospel, but not baptize? No one would have been saved.
Read that passage again. Why is Paul glad that he did not baptize the people there? Because he did not want to be additional fuel for the divisiveness that was in that congregation at that time. They were arguing that some were better than others (more saved? more righteous?) because they had been baptized by Apollos, or Cephas (Peter), or whomever else. Paul is saying that it doesn't matter by whom you were baptized. What matters is
into whose name you were baptized. Notice that Paul did baptize three people/families on his visit there. Why? Because those were the three people/families that believed the Gospel when he was first there. Others came to believe later when other ministers came to preach the Word. So as Paul says, he planted the Seed, Apollos and Peter and others watered the Seed, but it was God who gave the increase.
The Bible is clear that salvation is by grace through faith in Jesus Christ, not by works of any kind, including baptism see Ephesians 2:8-9. So, any interpretation which comes to the conclusion that baptism, or any other act, is necessary for salvation, is a faulty interpretation
That is a faulty interpretation of Eph 2:8-9. Because Rom 10:9-10, as mentioned earlier, clearly makes a physical action a prerequisite for receiving salvation. So Eph 2:8-9 cannot mean that there is no human action that in necessary to receive salvation, because that would contradict Rom 10:9-10. What it means is that there is no way to EARN salvation. We don't deserve salvation, and we can never make ourselves deserve it. It is a gift from God to His faithful servants.
Look at Luke 17:7-10. The servant (us) does not deserve even a "thank you" from his master (God) for doing what is commanded of him. But he is not really a servant if he doesn't do what the master commands. Then he is good for nothing but to be thrown out.
As to Mark 16:16 (I wish they would figure out who wrote the verses after verse 8)
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Mark 16:16 says, “the one who believes and is baptized will be saved,” but then says, “the one who does not believe will be condemned.” A person is condemned for not believing.
A person is condemned already because of his sin. He just remains condemned if he does not believe (John 3:18). So we go back to the first half to see how do we get out of being condemned. And that requires belief AND baptism. And whether this was part of the inspired writing or not, it agrees with 1 Pet 3:21, Acts 2:38, John 3:5, Gal 3:26-27, Eph 5:26-27, Acts 22:16, Acts 8:36, Rom 6:1-7, and Col 2:11-14.
It says nothing about being condemned because of not being baptized. This text should not be pressed to say what it does not say. The New Testament knows nothing of an unbaptized Christian, and therefore the text speaks generally about a baptized believer. It is not meant to deal with an unusual situation in which one believes and is not baptized.
One who is not baptized doesn't really believe in the first place, so the concept of an unbaptized Christ follower is an oxymoron.