One of the criminals admitted, "We are punished justly, for we are getting what our deeds deserve. But this man has done nothing wrong.” - they probably weren't preaching Christ seeing as he was right there. As for the "doing all they could do..."
2 Nephi 25:23
For we labor diligently to write, to persuade our children, and also our brethren, to believe in Christ, and to be reconciled to God; for we know that it is by grace that we are saved, after all we can do.
Ephesians 2:8-9
For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— 9 not by works, so that no one can boast.