Here in the original manuscript, Oliver Cowdery initially wrote “there was no God”. Later, probably when he read back the text to Joseph Smith, he corrected the was to is (his supralinear is is written with somewhat heavier ink flow). Oliver’s error here was probably influenced by the occurrence of “and I always knew that there was a God” in the immediately preceding verse. Here in verse 53, we have a direct quote, thus the present-tense is.
Summary: Maintain in Alma 30:53 the present-tense is, the corrected reading in 𝓞; here Korihor is directly quoting what the devil said to him.