This is what Nephi saw in vision. This is what he carved onto the Small Plates of Nephi (on which Enos was now writing). With these words Enos is accepting the prophecy of Nephi, and more likely also accepting the things which his "many long struggling" led him to realize.
Nephi knew his descendants would fail in their faith, be destroyed by the Lamanites, and that the Lamanites would also lose the true faith.
Given that Nephi wrote these things on the same plates on which Enos was now writing, the question arises as to why Enos puts his wording so tentative. He does not come right out and repeat what Nephi had written. Instead he makes this future uncertain. He writes: "[I]f it should so be, that my people, the Nephites, should fall into transgression, and by any means…" He knew this was going to happen. It is already prophesied in the very plates on which he was etching his own record. Given this, we should consider why he uses tentative language rather than definite language to describe this already known future. It was not because he was unaware of his people's fate. Nor was it due to a lack of faith on his part. Rather, since Enos would have been given the sealing power as a part of having his calling and election made sure, Enos' use of tentative language on something which has already been prophesied by Nephi is one of the confirming evidences that Enos has been given the sealing power (and correspondingly had his calling and election made sure). He did not want anything he said or wrote to bind the Lord to this unwanted outcome. Although he could accept this future and even call the enemy victors over his descendants his "brethren," he did not want to own any part of the responsibility for this fate.
Enos gives a display of the kind of character traits required for these most sacred things. People involved in such rites are able to honor sacred things and to handle with profound care the trust shown by the Lord. Enos shows us what we should all be trying to become. He has become sacred space in his very body, a Temple of God. He has become the place where the infinite and eternal touches the finite and mortal.