John Chrysostom

Here he clearly puts forward what I have stated: for the expressions "we were kept" and "shut up," signify nothing else than the security given by the commandments of the Law; which like a fortress fenced them round with fear and a life conformable to itself, and so preserved them unto Faith.

Homily on Galatians 3 ↗

Theophylact of Ohrid

The Law, he says, provided great safety to those who were under its protection, because it restrained them from many sins and was like a wall, surrounding people and leading them to faith. In what way? By exposing sins, but not having the power to free from them, it necessarily pointed to the justifying faith, which existed even in ancient times, but in a hidden manner, and was openly revealed later, when God also appeared in the flesh.

Commentary on Galatians ↗

Oecumenius

"before faith came." For indeed it did not afford the kind of security that would prevent most people from committing sins. "restrained." Being guarded, he says, toward the faith that was to come. How? For the law, which was exposing our sins but was not able to rescue, did nothing else than urge those who were more eager to run toward the faith that would be revealed for the deliverance from sins. …

Commentary on Galatians ↗

Thomas Aquinas

Then when he says, "But, before the faith came, we were kept under the law shut up," he gives experimental evidence of this service, as manifested in the case of the Jews. First, he states how the Jews were benefited; Secondly, he concludes a corollary (v. 24). He says therefore: If the scripture, i.e., the written Law, kept all things shut up under sin, what benefits did the Jews derive from the Law before faith came by grace? …

Commentary on Galatians ↗