Peter could have chosen the word chrestotes, a Greek term common in his day that carried the connotation of doing good. Instead he uses arete, a word that describes what we are, not what we do. This goodness precedes and also produces action.
But bad people can do good things. Jesus said that though we're sinful, we know how to give good gifts to our children (Luke 11:13). People don't have to be good to do good.
Peter aims at something entirely different. The virtue he calls us to acquire is goodness for nothing: good without needing reward, or prompting, or recognition. It's a quality of soul. In fact, it's what God is—absolute goodness. Just as Jesus told the rich young ruler that "no one is good but . . . God" (Mark 10:18 NKJV), no one can become good without Him.
Peter was as aware as Paul that knowledge for its own sake, as merely an intellectual trophy, is vainglory—a thing as prone to make us foolish as to make us smart. This knowledge Peter mentions isn't of the mind so much as it is of the heart. It's a knowing of more than a knowing about, relational more than rational.
Of course, in every relationship—whether it's with our co-worker or sister, or our Creator and Savior—we must have sufficient knowledge about the other if we're to have personal knowledge of the other. But beware theology without worship, or doctrine without intimacy and obedience. The knowledge Peter commends always leads to adoration and surrender. Simply put, to know God is to love God.
Peter likely wouldn't entirely grasp our popular understanding of this concept, where self-control has been reduced to a behavioral technique for anger management or weight loss.
Three times in his first letter, Peter mentions self-control, and each time eternal matters are at stake: we need self-control to understand the greatness of our salvation (1 Peter 1:10-13), to prepare ourselves for the end of all things (4:7), and to resist the onslaught of the Devil (5:8). It helps, incidentally, with waistlines and outbursts, but to be self-controlled is actually no less than to have the mind of Jesus, His clarity, His mental toughness, His focus on things above.
We tend to think perseverance is about willpower—gritting your teeth and pushing through. But Peter chose a military term that actually means "don't abandon your post." Perseverance is tied to obedience and rooted in calling.
It has nothing to do with the strength of your will, and everything to do with submitting your will to the will of another. Perseverance is refusing to quit because you're bored, endangered, or offended. All that is irrelevant. What matters is carrying your assignment to completion. Understood this way, we are in desperate need of perseverance.
Literal translation: good devotion. A wholehearted and unwavering commitment to God's character and ways. The enemy of godliness is worldliness, being captive to the world's values and habits and systems. The godly man or woman consistently chooses that which exalts God and reveals Christ—humility, purity, sacrifice, love.
6. Brotherly kindness.
The original word (philadelphia) has a narrow and technical meaning: to love the sons of my father. A man in the ancient world (and more and more, for different reasons, in our own world) might have several sons from different mothers (think of King David).
These different sons might lack deep natural affection one for another. But they love their father (hopefully), and for the sake of the father will practice philadelphia, love for each other. In short, philadelphia is loving what the Father loves, for the sake of the Father.
The word used here is agape: loving as the Father loves, by the power of the Father. Agape is more than loving our half-brothers and half-sisters. It's also loving the least of these (Matthew 25:45)—people "beneath" us whom we'd rather ignore; the most of these—people "above" us whom we're prone to resent; and the worst of these—people against us whom we're quick to despise. It's love for all—including enemies.
And that is why to love like this is more than any one of us can do in our own strength. We need God's transcendent, chastening, empowering and holy love to flow in us and through us so that we might love as He loves.