Ambrose of Milan

And so He Himself also chose Aaron as priest, that not the will of man but the grace of God should have the chief part in the election of the priest; not the voluntary offering of himself, nor the taking it upon himself, but the vocation from heaven, that he should offer gifts for sins who could be touched for those who sinned, for He Himself, it is said, bears our weakness. No one ought to take this honour upon himself but they are called of God, as was Aaron, and so Christ did not demand but received the priesthood.

Epistle 63 ↗

Bonaventure

A figure preceded in the rod and flower of Aaron. The history is in the rod of Aaron, which, cut off and dry, in the space of a single night put forth leaves, flowered, and brought forth fruit. Who brought it about that the Virgin conceived? Certainly the Holy Spirit, who is a love fervent, fruitful, undefiled, virile, incorrupt, and deifying. That he is a fruitful love is signified in the rod of Aaron, which in the space of a night put forth leaves, flowered, and brought forth fruit.

Collationes de Septem Donis, Collation 6 ↗