No problem here, right? Dominus, being a masculine noun, takes a masculine form of "one" (unus). Fides is feminine and so takes a feminine form of "one" (una). But what's up with baptisma? This sures looks to me like it's a feminine noun of the first declension in Latin. Then why is the word "one" neuter (unum)?
Here's the solution. Baptisma is a loan word from the Greek. And in Greek, baptisma is a neuter noun and it therefore requires a neuter "one." See?
Let's imagine a German speaker saying, "Mein Leben ist eine love-story." Some would call this Denglisch. But notice: even when using English words, the speaker was still thinking in German. The proof? They didn't say "ein love-story." They used the feminine eine because the word "story" in German, Geschichte, is feminine.
No language -- least of all English -- is immune to language borrowing. In New Testament Greek, we have words like Abba (Aramaic) and praitōrion (Latin). Or think of the way we use "wiki" in English ("wiki" is Hawaiian for "fast"). Unsurprisingly, then, baptisma entered Latin from the Greek but never lost its Greekishness (a word I just made up).
I'll close with this fantastic quote by James Nicoli: "English doesn't borrow from other languages. English follows other languages down dark alleys, knocks them over, and goes through their pockets for loose vocabulary."