Mark 16:16, "He that believeth and is baptized shall be saved; but he that believeth not shall be damned."
Unbelief is the unpardonable sin. Jesus DOES NOT say "he that is not baptized will be damned". Jesus DOES say "he that does not believe shall be damned."
Remember what Peter's confession was:
"But what about you? Who do you say I am?"
(Jesus is testing Peter's heart)
Simon Peter answered, "You are the Christ, the Son of the living God."
Peter was right on target on his confession of Jesus...as, also, we should be on our confession before we are baptized....