This will be a short post. This comes from sections 91-100 of the dialogue with Trypho. Justin teaches that sometimes people misread the Scriptures and think that God presented different forms of righteousness throughout history before Christ came. People from Abraham to Moses were righteous because of circumcision. People between Moses and Jesus were saved through circumcision and law. After Jesus, they are saved through grace in Jesus.
The truth is, all people in all history were saved through grace in Jesus. God called Abraham righteous, and then he gave him circumcision. His righteousness came before his work. God saved Israel from Egyptian slavery and then he gave them the Law. They were already saved and were called to act accordingly. And they all had the sign of God providing a substituted to sacrifice instead of Isaac. And they also saw the sign of looking to the brass serpent in the desert to be healed from their much-deserved snake bites.
They guys are starting to believe Justin but are still confused about the curse for people hanged on a tree. "Just as God commanded the sign to be made by the brazen serpent, and yet he is blameless; even so, though a curse likes in the law against persons who are crucified, yet no curse lies on the Christ of God, by whom all that have committed things worthy of a curse are saved." Christ suffered because of our disobedience, for our curse. And being perfect, he conquered death. The people were cursed because they did not follow the things written in the law according to Deuteronomy 27:26. Our perfect Jesus took that curse and ended it.