(Since there are not 3 chapters in 2 John, this “3:16” isn't an actual 3:16.)
6And this is love: that we walk according to His commands. This is the command as you have heard it from the beginning: you must walk in love.
2 John 6 (HCSB)
That sounds like circular reasoning, doesn't it?
"What is love?"
"Walk in His commands."
"Well, what are those commands?"
"Walk in love!"
It isn't actually circular reasoning - I believe that there are two different things we can take from this. The first of these reminds us of Jesus' words, when asked what the greatest commandment was.
34When the Pharisees heard that He had silenced the Sadducees, they came together in the same place. 35And one of them, an expert in the law, asked a question to test Him: 36“Teacher, which commandment in the law is the greatest?”
37He said to him, "Love the Lord your God with all your heart, with all your soul, and with all your mind. 38This is the greatest and most important commandment. 39The second is like it: Love your neighbor as yourself. 40All the Law and the Prophets depend on these two commandments."
Matthew 22:34-40 (HCSB)
Both parts of the greatest two commandments, according to our Lord, deal with love. This is the second principle we can see in this passage. Even looking back at the Ten Commandments, we can see this.
2I am the Lord your God, who brought you out of the land of Egypt, out of the place of slavery.
3Do not have other gods besides Me.
4Do not make an idol for yourself, whether in the shape of anything in the heavens above or on the earth below or in the waters under the earth. 5You must not bow down to them or worship them; for I, the Lord your God, am a jealous God, punishing the children for the fathers' sin, to the third and fourth generations of those who hate Me, 6but showing faithful love to a thousand generations of those who love Me and keep My commands.
7Do not misuse the name of the Lord your God, because the Lord will punish anyone who misuses His name.
8Remember to dedicate the Sabbath day: 9 You are to labor six days and do all your work, 10but the seventh day is a Sabbath to the Lord your God. You must not do any work - you, your son or daughter, your male or female slave, your livestock, or the foreigner who is within your gates. 11For the Lord made the heavens and the earth, the sea, and everything in them in six days; then He rested on the seventh day. Therefore the Lord blessed the Sabbath day and declared it holy.
12Honor your father and your mother so that you may have a long life in the land that the Lord your God is giving you.
13Do not murder.
14Do not commit adultery.
15Do not steal.
16Do not give false testimony against your neighbor.
17Do not covet your neighbor's house. Do not covet your neighbor's wife, his male or female slave, his ox or donkey, or anything that belongs to your neighbor.
Exodus 20:2-17 (HCSB)
Think about it - if you love the Lord, you won't have other gods, you won't worship idols, you won't misuse His name, and you'll set aside a day to remember what He has done for you. By the same token, if you love your fellow man, you'll honor your parents by recognizing what they've done for you; you won't murder your fellow man; you won't steal from them, whether their spouse or an earthly good; you won't lie about them; and you won't be jealous of the blessings they have received from God.
So, you see, it's not really circular at all. Love is the command, and the command is love. May God grow in us His love, both for Him, and our fellow man.