aretē

In ordinary Greek this was the word for whatever makes someone or something outstanding at being what it is. Writers used it of a soldier's courage, a citizen's public service, a horse's speed, even the fertility of good farmland. It does not name one particular good quality. It names the fact of being excellent in a way other people notice and applaud, which is why Philippians 4:8 sets it next to the praise it earns (ἔπαινος).

The New Testament uses this word very sparingly, and that restraint is worth noticing. Greek moral writers built entire systems on it, from Aristotle's catalog of virtues to the Stoic training program, but the New Testament writers usually reach for other language instead: being righteous, being holy, walking by the Spirit. When they do pick this word up, they use it the plain way, as a broad term for admirable character, without importing anyone's philosophical scheme.

In 2 Peter it becomes one link in a chain, where each quality grows out of the one before it: faith supplies excellence, excellence supplies knowledge, and so on until the chain ends in love. It is not a checklist there but a direction of travel, the picture of a faith that actually changes how a person lives.

The same word can point away from character and toward display. Greeks used it, often in the plural, for a god's celebrated acts or a wealthy benefactor's public services, the deeds a city carved on a monument so they would be remembered. The Greek Old Testament pushed the word further in this direction: it uses it to render the Hebrew word for praise (תְּהִלָּה) in Isaiah, where God says his people were formed to tell out his praise, and the Hebrew word for splendor (הוֹד) where God's majesty covers the heavens.

1 Peter 2:9 is drawing on that Isaiah language directly. When it says the church exists to proclaim these, the plural is not a list of God's good qualities. It is the record of what he has done, above all calling people out of darkness into his light. English versions divide over how to say it: "praises" points to the response, "excellencies" to the quality, "mighty acts" to the deeds themselves.

2 Peter 1:3 pairs the word with glory (δόξα) and applies both to God. Since glory in that pairing means greatness made visible, many readers hear this word alongside it as God's revealed power; others hear his moral excellence instead. The phrase is short and the two words are tied tightly together, so both readings stay live.