Theophylact of Ohrid

I, He says, not only do not forbid the one who performs miracles in My name, but if anyone gives you anything, even the very least, for My sake and not for the sake of worldly people, even that person will not lose his reward. And He spoke of the cup of water with people in mind who make excuses of poverty. If, He says, you give even a cup of water — and nothing less than this is possible — even this will not be lost to you. …

Commentary on Mark ↗

Augustine of Hippo

By which He shows, that he of whom John had spoken was not so far separated from the fellowship of the disciples, as to reject it, as a heretic, but as men are wont to hang back from receiving the Sacraments of Christ, and yet favour the Christian name, so as even to succour Christians, and do them service only because they are Christians. …

Catena Aurea by Aquinas ↗

Bede

For whoever gives you a cup of cold water to drink because you belong to Christ, truly I tell you, he will not lose his reward (Psalm 140). We read in the prophet David: To excuse excuses in sins, that many presented quasi-just excuses for their sins, so that they seem to sin out of necessity what they willfully commit. The Lord, the searcher of hearts and minds, who foresees future thoughts in each individual, had said: Whoever receives one such child in my name receives me. …

On the Gospel of Mark ↗