Gregory the Dialogist

Since it is evident to all, dearest brothers, that our Redeemer came into the world for the redemption of the nations, and since we see Samaritans being called to faith daily, what does it mean that when sending the disciples to preach, He says: "Do not go into the way of the Gentiles, and do not enter into the cities of the Samaritans, but go rather to the lost sheep of the house of Israel"? …

Forty Gospel Homilies, Homily 4 ↗

Hilary of Poitiers

The promulgation of the Law deserved also the first preaching of the Gospel; and Israel was to have less excuse for its crime, as it had experienced more care in being warned. Though they are here called sheep, yet they raged against Christ with the tongues and throats of wolves and vipers.

Catena Aurea by Aquinas ↗

Thomas Aquinas

But go rather to the lost sheep of the house of Israel. Why sheep? Because they were lost more by the fault of the Pharisees than by their own. Hence Psalm 90 (v. 3): "We are his people and the sheep of his pasture" and 1 Peter (2:25): "You were straying like sheep, but now have returned to the shepherd and guardian of your souls." But what will they do, as they go about? Preach as you go: "I have appointed you to go and bear fruit, fruit that remains" (Jn 15:16).

Commentary on Matthew ↗