The original manuscript is not extant here, but Oliver Cowdery, in his copying from 𝓞 into 𝓟, initially wrote things, then erased the plural s. The erasure shows that the change was immediate. Moreover, the use of the singular pronoun it at the end of the passage supports the singular thing, although one could interpret the change from things to thing as a case of editing (that is, as an attempt to make thing agree with the following it). A similar error is found in 1 Nephi 15:11. There the original manuscript is extant and reads “do ye not remember the thing which the Lord hath said”. There too Oliver initially wrote things as he copied from 𝓞 into 𝓟, and then he immediately erased the plural s ending. This example suggests that the same thing occurred in Jacob 7:14: Oliver probably corrected the printer’s manuscript to make it agree with the reading of the original manuscript. For additional examples of the scribal tendency to mix up things and thing, see under 1 Nephi 15:11.
Summary: Maintain in Jacob 7:14 the singular thing (“in the thing which thou knowest to be true”), Oliver Cowdery’s corrected reading in 𝓟 and the probable reading of 𝓞.