Theophylact of Ohrid

Or else, the lord of the house is the intellect, which points out the large upper room, that is, the loftiness of intelligences, and which, though it be high, yet has nothing of vain glory, or of pride, but is prepared and made level by humility. But there, that is, in such a mind Christ's Passover is prepared by Peter and John, that is by action and contemplation.

Catena Aurea by Aquinas ↗

Bede

Follow him, and wherever he enters, say to the master of the house, that the teacher says: Where is my guest room, where I may eat the Passover with my disciples? By referring to both the water-bearer and the master of the house, the omitted words are suggesting that, to all wishing to celebrate the true Passover, that is, to be imbued with the sacraments of Christ and to receive him in the hospitality of their minds, this opportunity is indicated.

On the Gospel of Mark ↗