For truly, I say to you,
whoever gives you a cup of water to drink because you belong to Christ
will by no means lose his reward.
This promise from Jesus is triggered by the disciple John telling Jesus that the disciples had seen a man casting out demons in Jesus' name, and they tried to stop him because the demon caster was not in the disciple group ("He was not following us"). Jesus, however, issues a reprimand, saying, "the one who is not against us is for us." He also says "no one who does a mighty work in my name will be able soon afterward to speak evil of me. "
One of the implications of this passage is that God is at work in the world in ways often beyond our comprehension. All kinds of heathens and pagans participate in feeding, clothing, and taking care of each other, generally behaving as well as Christians in many ways in everyday life. If these efforts do not make Christian life more difficult, they should not be hindered. In fact, they should be thanked and encouraged, and simply witnessed to that they are actually pleasing to a God they do not know -- "How would you like to know him and the full gamut of blessings he is offering to you?" So anyone offering even a cup of water will not lose his reward. Notice how low the bar is set here for defining a good work.
Is there really a reward for doing a good work in the world? Of course there is. In the Old Testament, the conclusion to the giving of the laws include a promise to "live long and prosper in the land I will give you." We all know that when people refrain from stealing, lying, covering, adultery, killing, and defying appropriate authority, life goes more smoothly and pleasantly for everybody. Likewise, offering a cup of water to a thirsty person makes life better.
The real problem is stated in the next verse, however. “Whoever causes one of these little ones who believe in me to sin, it would be better for him if a great millstone were hung around his neck and he were thrown into the sea." Jesus is really clear. Help his people-- you don't lose your reward; hinder his people, scatter his people, give his people pause -- be prepared for the worst.
There is a call here to discern our actions. Are we helping or hindering? There is also the further call to recognize our overwhelming need for a redeemer to ransom us from the penalties we have earned when we have failed to help, or actively hindered his people.
The world is full of risks, rewards, and penalties, but fortunately Jesus has overcome the penalties of the world --
For I am sure that neither death nor life, nor angels nor rulers, nor things present nor things to come, nor powers, nor height nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord. (Romans 8:38-39)
Thanks be to God for Whose we are!