38 John said to him, “Teacher, we saw someone casting out demons in your name,[f] and we tried to stop him, because he was not following us.” 39 But Jesus said, “Do not stop him, for no one who does a mighty work in my name will be able soon afterward to speak evil of me. 40 For the one who is not against us is for us. 41 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.
Each person enters a saving relationship with Jesus when that person responds to grace and chooses to believe in Him. No one will ever be saved by works. That being said, the most telling evidence of what a person believes is what a person does. The disciples encountered someone who was doing something. They later told Jesus, “We told this guy to knock it off because he was not a member of our denomination.” Jesus took the opportunity to make a vital point – every person is either with Him or against Him. The man the disciples confronted could not have done a miracle in the name of Jesus without the power of Jesus. Jesus then spoke to the consistency that must exist between words and deeds. Talking about Jesus is the easy part; actually doing something is the hard part.
As far as the cup of water goes, there needs to be a clear distinction: Jesus did not say that merely giving a cup of water netted a reward, but giving a cup of water because of Him. You could make a case that theologically conservative churches and movements tend to become too politicized. At the opposite end of the spectrum are the liberal churches and movements that abandon or dilute spiritual truth and solely focus on temporal needs. Jesus calls us to show compassion without neglecting spiritual truth.