The word translated truly (or verily) is the Hebrew, and then Greek word, amen. For the last three millennium amen has been used in a liturgical fashion by Jewish people in response to a blessing. It means “we agree with this, we receive this as God’s truth for us.”
Amen is a required conclusion of any Christian prayer. If someone publicly prays and we don’t hear an “amen” we wonder it the prayer still has more to say. (Though for some curious reason, I have a son-in-law who refuses to conclude his prayers with either an, “In Jesus name” or an “amen.”)
When my son who was sixteen was wondering about the truth of his parents’ faith, I asked an atheist friend to talk with my son and tell him exactly why he came to the conclusion that Christianity was not true. He looked at me like I was some sort of an abusive father. “I would never do that,” he said. "Can you even begin to understand the suffering and the rejection I’ve experienced since I left the faith?” I expressed sympathy for his suffering but I said, “The truth is the truth. If the belief system in which we raised our son is not valid, I want him to have the opportunity to find that out. “
“I would never do that to your son,” he told me.
Our Judeo-Christian heritage puts an emphasis on truth unique among religious systems. The truth may hurt, excruciatingly at times. But even the harshest truths can work out for the good, for God is good.
To that second thief on the cross Jesus said, “Truly, I tell you today, you will be with me in paradise.”
I’m guessing that second thief was Jewish. So did he then respond to what Jesus said to him with an amen?