The subordinate conjunction that was omitted by the 1874 RLDS edition and restored to the RLDS text in the third RLDS edition (1908). Either reading, with or without the that, is theoretically possible in English, although in almost every instance in the Book of Mormon text when the verb believe takes a finite clause as its complement, the subordinate conjunction that precedes the clause (67 times). In one place, the that was omitted when Oliver Cowdery copied the text from 𝓞 into 𝓟:
In this instance, the 1830 edition restored the that, apparently by reference to the original manuscript (𝓞 was used to proof the 22nd gathering of the 1830 edition, covering Alma 41:8–46:30). See the discussion accompanying plate 11 of volume 1 of the critical text.
There is only one instance in the earliest text where there is no that before the finite-clause complement of the verb believe:
In this instance, the 1837 edition added the that, with the result that now in the current LDS and RLDS texts there is no example of the verb believe where its finite-clause complement lacks the subordinate conjunction that. The critical text will in each case follow the earliest textual sources, thus removing the intrusive that in Alma 32:25 (for further discussion, see under that passage). For a general discussion of the use of the subordinate conjunction that after other verbs, see under that in volume 3.
Summary: Maintain the normally expected that after believe in Alma 19:9: “I believe that it shall be according as thou hast said”.