Here the printer’s manuscript had life, but the 1830 typesetter misread life as light. The 1908 RLDS edition and the 1981 LDS edition restored the original life.
There are numerous references in the scriptures to eternal life, but there is no verse that specifically refers to “eternal light”. In addition, the Gospel of John refers to Christ in terms of both life and light but indicates that in the Son was life (just as here in Ether 3:14):
But Grant Hardy points out (personal communication, 16 January 2007) that there are three references to “everlasting light” in the scriptures, including one in the Book of Mormon:
Thus the idea of eternal light is possible. In fact, there is one verse that contrasts “endless light” with “endless life”:
So either reading is theoretically possible in Ether 3:14, and we therefore follow the earliest reading, “in me shall all mankind have life and that eternally”.
Hardy also points out that the source for the 1830 change to light here in Ether 3:14 may have been the six occurrences of light in the preceding text:
Hardy also considers the possibility that the 1830 typesetter, John Gilbert, consciously emended life to light here in Ether 3:14, although that seems unlikely since Gilbert made emendations only when something was clearly wrong with the text; there is nothing wrong with life here in Ether 3:14. Nor is it reasonable to think that life itself (the reading in 𝓟) might be an early error for light, mainly because there are no preceding occurrences of life that could have triggered such an error (the nearest preceding occurrence of life is in Mormon 8:39). For another example where an original life was apparently changed to light under the influence of numerous preceding instances of light, see under Alma 19:6.
Summary: Maintain in Ether 3:14 the earliest reading of life; usage elsewhere in the scriptures supports the occurrence of life in this passage; the 1830 reading with light seems to have been prompted by the preceding instances of light in Ether 2–3.