A.T. Robertson 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)

Because ye are Christ's (οτ Χριστου εστε). Predicate genitive, belong to Christ. See Ro 8:9; 1 Corinthians 1:12; 2 Corinthians 10:7. That is the bond of universal brotherhood of the redeemed. It breaks over the lines of nation, race, class, sex, everything. No service is too small, even a cup of cold water, if done for Christ's sake. See on Mt 18:6f. for discussion on stumblingblocks for these little ones that believe on Jesus , a loving term of all believers, not just children.