Oliver Cowdery initially wrote these things; then almost immediately he corrected this phrase to this by crossing out the final e of these and the following things and then overwriting the first e in these with an i (the level of ink flow is unchanged). One possible source for this scribal error is in the previous verse, where it reads “after ye have known and have been taught all these things” (Mosiah 2:36). There would have been no motivation to emend these things to this in the next verse except that the original manuscript read this.
Summary: Retain in Mosiah 2:37 Oliver Cowdery’s virtually immediate correction, the word this (most probably the reading of the original manuscript).