Whosoever shall give you a cup - How easy it is to be a Christian! What is easier than to give a cup of cold water to a thirsty disciple of Jesus! But it must be in his name - that is, because he "is" a Christian, and therefore from love "to the Saviour." This is very different from giving it from a mere motive of common kindness. If done from love to Christ, it will be rewarded; and hence we learn that the humblest acts of Christians - the lowest service that is rendered - will be graciously noticed by Jesus and rewarded. None are so humble in his kingdom as not to be able to do good, and none so poor that he may not show attachment to him. The feeblest service will be accepted, and acts of love that may be forgotten by man, will be remembered by Him, and rewarded in heaven.

A cup of water to drink - See the notes on Matthew 10:42; Matthew 18:6-8.

For whosoever shall give you a cup of water to drink,.... Not only one that does a miracle in the name of Christ, but he that shows the least respect, or does the least kindness to any one of his; See Gill on Matthew 10:42; is to be reckoned a friend, and is so accounted by Christ; and will be sooner or later taken notice of by him, especially, if what he does, be it ever so little, is done on his account:
in my name, because ye belong to Christ; or as the Syriac version renders it, "on account that ye are Christ's"; are his disciples, are called by his name, are partakers of his grace, bear his image and likeness, are loved by him, interested in him, given to him, redeemed by him called by his grace, and shall be with him:
verily l say unto you, he shall not lose his reward; See Gill on Matthew 10:42.

It is repeatedly said of the wicked, Their worm dieth not, as well as, The fire is never quenched. Doubtless, remorse of conscience and keen self-reflection are this never-dying worm. Surely it is beyond compare better to undergo all possible pain, hardship, and self-denial here, and to be happy for ever hereafter, than to enjoy all kinds of worldly pleasure for a season, and to be miserable for ever. Like the sacrifices, we must be salted with salt; our corrupt affections must be subdued and mortified by the Holy Spirit. Those that have the salt of grace, must show they have a living principle of grace in their hearts, which works out corrupt dispositions in the soul that would offend God, or our own consciences.

For whosoever shall give you a cup of water to drink in my name, because ye belong to Christ, verily I say unto you, he shall not lose his reward--(See on Matthew 10:42).
Continuation of Teaching Suggested by the Disciple's Strife (Mark 9:42-50).
What follows appears to have no connection with the incidental reproof of John immediately preceding. As that had interrupted some important teaching, our Lord hastens back from it, as if no such interruption had occurred.

For whosoever shall give you a cup - Having answered St. John, our Lord here resumes the discourse which was broken off at Mark 9:37. Matthew 10:42.