In Mosiah 16:6, Abinadi said, "And now if Christ had not come into the world, speaking of things to come as though they had already come, there could have been no redemption." This usage of the past tense is known as the prophetic perfect tense. In Mosiah 14 (Isaiah 53), for example, where Isaiah is speaking messianically, he uses present, future, and past tense verbs! One thing that is rather confusing about reading Isaiah is that the translations flip around from past, to present, and future. We often cannot tell whether Isaiah is giving a report of something that actually had happened to a servant who was treated badly, something that is currently happening, or something that is yet to occur.
This problem is caused by the fact that Hebrew verbs are not always crystal clear in their tense. Sometimes the verbs are not even expressed, so they and their tenses have to be implied from their contexts. But, in fact, it was common for prophets to speak about future things in the past tense. Even though this may seem odd, Hebrew grammar texts call this the prophetic past or prophetic perfect. Perhaps this is because, once the prophets had the future revealed to them, it was as though they had seen the preview of the coming attraction, and now they were talking to their audiences about what had happened in the preview. They were talking about what they had seen spiritually in the vision, but they expressed it in the past, even though it had not yet occurred physically and temporally.
There is no better description of the prophetic past than the one here in Mosiah 16:6, when Abinadi says that he is "speaking of things to come as though they had already happened." We do not know who put that little aside in there. Abinadi, when he said, "And now if Christ had not come into the world," probably did not stop and say, "Oops, let me just remind you that I am talking about things as if they have happened but they are still to happen." Maybe he did, but it may also have been Alma the Elder, or Alma the Younger as an editor, or Mormon as an abridger, or someone else saying, "This was expressed in the past tense, but I know that the meaning is future." In any event, that is how the prophetic perfect actually works.
Book of Mormon Central, "Why Did Book of Mormon Prophets Speak of Future Events as if They Had Already Happened? (Mosiah 16:6)," KnoWhy 95 (May 9, 2016).