To use a definite article is to say to your audience: “I know what I’m talking about and I think you know what I’m talking about, too.” Your audience can correct you and say, “Nope, no clue. I can’t identify what you’re talking about.” But if they don’t, you as a speaker feel comfortable proceeding with whatever you’re going to say.
So in John 1:18: where the author writes: ἡ χάρις καί ἡ ἀλήθεια διὰ Ἰησοῦ Χριστοῦ ἐγένετο, he’s expressing confidences that his audience knows what he’s talking about with χάρις and ἀλήθεια. Now, yes, in Greek abstract nouns have a tendency to be articular. That is certainly true, but that ignores another pattern in the language: if you want to talk about something in particular, you introduce it first, without the article, to establish it in the minds of your audience and then after that, you can proceed to talk about it with the article.
And lo and behold in John 1:14, the author has already done precisely that:
- Καὶ ὁ λόγος σὰρξ ἐγένετο καὶ ἐσκήνωσεν ἐν ἡμῖν, καὶ ἐθεασάμεθα τὴν δόξαν αὐτοῦ, δόξαν ὡς μονογενοῦς παρὰ πατρός, πλήρης χάριτος καὶ ἀληθείας.
In the first reference to χάριτος and ἀληθείας, John couldn’t count on his audience being able to identify them and their relation to Jesus, so no definite article. But once introduced, he can then proceed on the assumption of shared knowledge with his audience and thus χάριτος and ἀληθείας each get the article in verse 16.
Take a look for yourself. Find a noun with an article and see if you can trace back to where the idea, person, or thing it refers to was first introduced. It probably won’t have an article at that point. There are other principles that often guide usage, but this one is probably the most basic and pervasive.