Cyril of Jerusalem

Of the robbers crucified with him, it was said: "He was reckoned among the wicked." Up to this time both were wicked, but one of them was wicked no longer. For one was wicked to the end, yielding not to salvation, and, though his hands were fastened, he struck blasphemously with his tongue.

Catechetical Lecture 13:30 ↗

Theophylact of Ohrid

The Lord was also crucified with the robbers so that people would think ill of Him, that He too was an evildoer. But this was by God's dispensation, for on the one hand the prophecy was fulfilled: "He was numbered with the transgressors" (Isa. 53:12), and on the other hand the two robbers were figures of the two peoples — the Jewish and the Gentile. Both of these peoples were lawless — the Gentile, as having transgressed the natural law, and the Jewish, as having transgressed both this law and the written law that God had given them. …

Commentary on Mark ↗

Augustine of Hippo

Nevertheless even the cross itself, if thou considerest it well, was a judgment-seat; for the Judge being set up in the middle, one thief who believed was delivered, the other who reviled was condemned. Already He signified what He is to do with the quick and the dead: some He will set on His right hand and others on His left. That thief was like those that shall be on the left hand, the other like those that shall be on the right. He was undergoing judgment, and He threatened judgment.

Tractates On John 31.11 ↗

Bede

And with Him they crucified two robbers, one on His right, and the other on His left. The robbers who are crucified with the Lord signify those who, under the faith and confession of Christ, undergo either the struggle of martyrdom or any other practices of stricter continence. But whoever undertakes these solely for the eternal and heavenly glory, these are undoubtedly identified by the merit and faith of the right-hand thief. …

On the Gospel of Mark ↗