(Since 3 John does not have 3 chapters, this “3:16” isn’t an actual 3:16.)
5Dear friend, you are showing your faith by whatever you do for the brothers, and this you are doing for strangers;— 3 John 5 (HCSB)
The book of 3 John is a letter written from John to Gaius, a believer who was doing good things. A group of believers, led by Demetrius, was visiting in his area, and John wrote this letter to commend Gaius on how he was helping them. Helping other people, especially those you don’t know and those doing God’s work, is a recurring theme throughout the Bible.
When Jesus was sending out His disciples, He proclaimed a blessing for those who would welcome then and support them.
40"The one who welcomes you welcomes Me, and the one who welcomes Me welcomes Him who sent Me. 41Anyone who welcomes a prophet because he is a prophet will receive a prophet’s reward. And anyone who welcomes a righteous person because he’s righteous will receive a righteous person’s reward. 42And whoever gives just a cup of cold water to one of these little ones because he is a disciple - I assure you: He will never lose his reward!"— Matthew 10:40-42 (HCSB)
Just a cup of water! That’s not hard at all, yet Jesus said that it will be rewarded. In Hebrews 13, the author encourages hospitality.
1Let brotherly love continue. 2Don’t neglect to show hospitality, for by doing this some have welcomed angels as guests without knowing it.— Hebrews 13:1-2 (HCSB)
Would you give an angel a hand if you could? By helping others and being hospitable, you may actually have the ability to help an angel! And, even if you never come into contact with one (and I’m not really sure how you would know even if you did), by being hospitable to others, you can do the work of an angel for them.
Finally, let’s look at an Old Testament example of hospitality.
8Then the word of the Lord came to him: 9"Get up, go to Zarephath that belongs to Sidon, and stay there. Look, I have commanded a woman who is a widow to provide for you there." 10So Elijah got up and went to Zarephath. When he arrived at the city gate, there was a widow woman gathering wood. Elijah called to her and said, “Please bring me a little water in a cup and let me drink.” 11As she went to get it, he called to her and said, “Please bring me a piece of bread in your hand.”
12But she said, “As the Lord your God lives, I don’t have anything baked - only a handful of flour in the jar and a bit of oil in the jug. Just now, I am gathering a couple of sticks in order to go prepare it for myself and my son so we can eat it and die.”
13Then Elijah said to her, “Don’t be afraid; go and do as you have said. Only make me a small loaf from it and bring it out to me. Afterwards, you may make some for yourself and your son, 14for this is what the Lord God of Israel says: ‘The flour jar will not become empty and the oil jug will not run dry until the day the Lord sends rain on the surface of the land.’”
15So she proceeded to do according to the word of Elijah. She and he and her household ate for many days. 16The flour jar did not become empty, and the oil jug did not run dry, according to the word of the Lord He had spoken through Elijah.— 1 Kings 17:8-16 (HCSB)
This kind lady was willing to give up nearly all the food she had in her house to help Elijah; yet, her kindness was rewarded by God’s provision over many, many days. And, in the remainder of this chapter, 1 Kings 17:17-24, her son dies, but is then raised by God through Elijah! That’s quite a reward, just for the simple act of giving up a little food.
“Random acts of kindness” is the idea that you do something nice for someone else, with the hope that someone else will do something nice for you. Mysticism has the idea of “karma,” where the things you do (for good or bad) determine how the rest of the world treats you. Neither of these come from the Bible - we should do nice things for other people “just because,” not to get a reward; and blessings from doing good come from God, not some nebulous “spiritual balance” that we can manipulate.
However, the prevalence of this theme, both in the Bible and outside It, show us that God blesses those who bless others in His name. Look for opportunities to bless others - you never know who you may be helping!