Brian, this is going to be complicated, so I apologize in advance.
Look at 1 Peter 3:20-21. These verses are often used to justify salvation by baptism, or that at the very least baptism plays a part in our salvation.
However, I want you to also look at Hebrews 11:7, where it is stated plainly and unequivocably that it was the ark that saved Noah and his family.
The only part that water played was the washing away of the sinful--just as Christ's blood washes away our sins.
Baptism doesn't put away the filth of our flesh, it puts on a good conscience toward God (1 Peter 3:21) by the resurrection of Christ (which coincides with Romans 6:4, which plainly states that baptism is the likeness of Christ's death, burial, and resurrection).
Now, Jesus did plainly state that he who believes and is baptized shall be saved; but what people tend to gloss over and ignore is what Jesus plainly stated immediately after (and it's really part of the preceeding statement, making it a full sentence): "But he who believes not shall be damned.
Did Christ make a mistake here? Shouldn't that read "he who believes not and isn't baptized shall be damned"?
Or is there possibly a halfway point? Those that believe, but aren't baptized, aren't actually damned, but maybe they aren't saved, either?
That, my friend, is the problem with those who place sole emphasis on "he who believes and is baptized shall be saved." In order to make baptism a part of the salvation process, you HAVE to ignore the rest of the sentence--because the rest of the sentence places the emphasis on simply believing, and leaves out baptism.
John the Baptist echoed this himself in John 3:36, when he talked about "he that believes" and "he that believes not." No mention of baptism.
The apostle John echoed it as well in John 20:31 -- But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name. Again, no mention of "believe and be baptized."
You made mention of Matthew 28; however, Matthew's record makes no mention of "believe and be baptized." Instead, it says "go and teach all nations, baptizing them." Your reference was in the context of a previous discussion we had, regarding baptizing in the name of the Father, the Son, and the Holy Ghost; it does not necessarily apply to this conversation.
The instructions you're talking about in Acts 2:38 are "repent and be baptized."
The fact of the matter remains: The only promise we are given in all 66 books of the Bible is the promise of salvation. The only way we are told that we might receive that free gift is through our acceptance--our belief--in Jesus Christ and His finished work on the cross.
It cannot be called a gift if we have to do anything--ANYTHING--to receive it. And it is most certainly called a gift more than once.
If we must speak in tongues to evidence it to others, it is like the child who says "I got a diamond ring from my father," and no one believes the child until she runs home and gets the ring to show them. Whether the child returns with the ring or not, the other children have judged her and refuse to believe her until she shows the evidence of her claim. Their refusal to believe her claim in no way diminishes the fact that she was given a gift from her father; if she fails to return and show them the proof, they will nod their heads and smugly say to each other, "she didn't really receive a gift from her dad." This also in no way diminishes the fact that she received a gift from her father.
Beware, my friend, of those that teach that you must show evidence of the existence of God, either in yourself, in others, or in the universe. If you want to see what I mean by this warning, the next time one of your Oneness Pentecostal brethren ask or urge or otherwise encourage you to "exhibit" a gift of the Holy Spirit (without, of course, you yourself being led by the Holy Spirit to do so), refuse. Do so kindly, without malice. Tell them you are not led by the Spirit at that time to do so.
Do that once, and they'll be disappointed. Do it twice, and they'll be suspicious. Do it more than twice, and you'll receive a visit.
Sorry about the length.