John Chrysostom

"For if the first-fruits be holy, the lump also is holy; and if the root be holy, so are the branches;" So calling in this passage by the names of the first-fruit and root Abraham, and Isaac, and Jacob, the prophets, the patriarchs, all who were of note in the Old Testament; and the branches, those from them who believed. Then since the fact met him that many had disbelieved, observe how he undermines it again...

Homily on Romans 19 ↗

Theophylact of Ohrid

He calls the patriarchs the "firstfruit" and the "root," and those of their descendants who believed the "lump" and the "branches." So he comforts the unbelieving Jews, saying: you too will be holy if you believe. For it is necessary that the lump and the branches be like the firstfruit — that is, the former like the firstfruit, and the latter like the root. But if you have not become like them, this is a sign of great wickedness.

Commentary on Romans ↗

Irenaeus

And that the Saviour received first-fruits of those whom He was to save, Paul declared when he said, "And if the first-fruits be holy, the lump is also holy," teaching that the expression "first-fruits" denoted that which is spiritual, but that "the lump" meant us, that is, the animal Church, the lump of which they say He assumed, and blended it with Himself, inasmuch as He is "the leaven."

Against Heresies Book I ↗

Thomas Aquinas

Then when he says, for if the firstfruit, he proves the same thing by considering the status of the Jewish race. This he does in two ways. First, on the part of the apostles, when he says: if the firstfruit be holy, so is the lump also. That is called the representative portion which is taken from the whole lump of dough, as if for approval. The apostles chosen by God from the Jewish people are the representative sample taken from the whole lump. …

Commentary on Romans ↗