"He is an Orthodox Christian."
Translation:Αυτός είναι ένας ορθόδοξος χριστιανός.
Why is the sentence without the article also marked correct? I admit to asking out of frustration; at times duolingo seems to be overly pedantic that you provide THE EXACT right answer, with not a frill attached. But other times it seems like you can add a word, drop a word, it doesn't really matter. But the problem is that there seems to be no way to divine when to be precise and when.
There is actually a (loose) rule to using indefinite articles in Greek, that's why.
If, in the English translation, you would never say 'two', then you don't really need to use the indefinite article in Greek, and most Greek speakers wouldn't in regular discourse. Indefinites aren't actually used that frequently in Greek.
Another way of thinking about it is that if there can be no confusion over quantity, then you don't really need to use the indefinite.
In the above example, the man is never going to be two Orthodox Christians, so therefore, you don't need to use the indefinite article.
But, if the man wanted an apple, you'd need to use it, because he could conceivably want two or three or four apples.
I actually think the best translation of this sentence would omit the ένας.