Here in the original manuscript, it appears that Oliver Cowdery initially wrote something like “and thus they did obtain a hope through faith unto eternal salvation”, although we cannot be sure that the word was obtain since only the end of the word is extant in 𝓞:
There is support for obtain as the initially written word in 𝓞 since it is found with the noun hope elsewhere in the text:
Moreover, there is independent manuscript evidence that Oliver sometimes wrote obtain in place of retain:
There are two other possibilities for the initial error in Alma 25:16: maintain and attain. In the footnote to my transcript of 𝓞 for this passage (see volume 1 of the critical text), I suggested that Oliver initially wrote maintain in this passage. Ultimately, of course, the reading of the original text is retain, the corrected reading in 𝓞, not whatever Oliver initially wrote in 𝓞.
Summary: Maintain in Alma 25:16 the corrected reading in 𝓞: “and thus they did retain a hope through faith unto eternal salvation”.