In the printer’s manuscript, Oliver Cowdery initially wrote “of things pertaining to Christ”; then somewhat later he supralinearly inserted the definite article the before things (the ink flow for the the is slightly heavier). The 1830 edition also has “of the things”, so the word the was undoubtedly in the original manuscript.
Elsewhere in the text, there are four examples with the before things in phrases with the form “pertaining (un)to X” and ten examples without the the, so it is not surprising that Oliver Cowdery initially omitted the the here in 3 Nephi 6:23. In fact, we have noun phrases of this type that differ in the use of the definite article, as in specific references to “things pertaining (un)to righteousness”:
Although the occurrence of the in these instances is infrequent, there is no reason to reject it. The critical text will maintain the use of the before things here in 3 Nephi 6:23.
Summary: Accept in 3 Nephi 6:23 the definite article the in “of the things pertaining to Christ”; not only is the possible here, but it is also the reading in the 1830 edition as well as the corrected reading in 𝓟.