In copying from 𝓞 to 𝓟, Oliver Cowdery accidentally replaced the determiner that with the indefinite article a, even though this “great and abomination church” has already been extensively referred to (in 1 Nephi 13:4–9). The 1981 LDS edition restored the that.
Summary: Maintain the definite determiner that in 1 Nephi 13:26 (the reading of the original manuscript); the great and abominable church has already been referred to, so the indefinite article a that Oliver Cowdery introduced is inappropriate.