Because ye are Christ's (οτ Χριστου εστε). Predicate genitive, belong to Christ. See Romans 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 Matthew 18:6 for discussion on stumbling-blocks for these little ones that believe on Jesus (Mark 9:42), a loving term of all believers, not just children.