Here the 1902 LDS edition accidentally added the definite article the, giving “descending out of the heaven” rather than the original “descending out of heaven”. The Book of Mormon uniformly has heaven without the definite article the whenever the text refers to beings coming down from heaven or going up to heaven (30 times, counting this one in 1 Nephi 12:6). We have, for instance, these examples with the phraseology “out of heaven”, of which four are preceded by the present participle descending:
There are a few cases where the textual history has “the heaven”. One case involves an example of scribal error that was immediately corrected:
Another case has the reading “in the heaven above” in the earliest textual sources:
But this passage is one of the Ten Commandments (found in Exodus 20:4), which reads “in heaven above” in the King James Bible. When Mosiah 13:12 also quotes this commandment, it reads like the biblical source (“in heaven above”). Thus “in the heaven above” in Mosiah 12:36 may very well be an error.
There is one case where the original manuscript apparently read “in the heaven”:
Here 𝓟 and the 1830 edition are each a firsthand copy of 𝓞, and both read the same. The language of this passage suggests the phraseology of Genesis 1:1 (“in the beginning God created the heaven and the earth”). Thus the use of “the heaven and the earth” is probably correct in Mormon 9:17.
For a complete discussion, see each of these three passages (Mosiah 2:41, Mosiah 12:36, and Mormon 9:17). Here in 1 Nephi 12:6, it is quite clear that the earliest reading is the correct one.
Summary: Maintain the phraseology “out of heaven” in 1 Nephi 12:6; this reading is supported by the earliest textual sources as well as by usage elsewhere in the text.