John Gill Commentary Mark 9:41

"For whosoever shall give you a cup of water to drink, because ye are Christ`s, verily I say unto you, he shall in no wise lose his reward." — Mark 9:41 (ASV)

For whoever gives 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 you belong to Christ ;
or as the Syriac version renders it, "on account that you 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:

truly I say to you, he will not lose his reward ;
(See Gill on Matthew 10:42).