The mistake you continue to make is denying what the Baptist actually said. According to him the reference was not in regard to what would happen in the future. Now let us look at the verse again:
"The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which beareth the sin of the world" (Jn.1:29).
In what sense did the Lord Jesus bear the sins of the world? It was by bearing the consequences of the sins of the world. Here is an example of the phrase "bear your iniquities" referring to bearing the consequences of iniquities:
"After the number of the days in which ye searched the land, even forty days, each day for a year, shall ye bear your iniquities, even forty years, and ye shall know my breach of promise" (Num.14:34).
The consequences of the iniquities of the Israelites was that they were not to enter Canaan for forty years. So when it is said that they were to bear their iniquities the meaning is that they were to bear the consequences of their iniquities. So when the Baptist spoke of the Lord Jesus bearing the sins of the world he was referring to the fact that while on the earth the Lord Jesus would bear the consequences of the sins of the world.
Despite these facts you can somehow trick your mind into believing that the Baptist was speaking about what would happen in the future despite the fact that he was speaking about something which had its fulfillment in the present.
You need to learn to deal with the Scriptures AS THEY ARE WRITTEN instead of the way that you think that they should be written.