Why did one person of the Trinity have to become a Redeemer?
Because the triune God, in eternity past, foreknew that the human race would fall in Adam. By the determinate agreement of the Father, Son, and Holy Spirit, their predeterminate counsel required that one person of the Godhead become the Redeemer.
Les explains that way back in eternity past, God the Father, God the Son, and God the Holy Spirit came together and, by a determinate agreeing of their three minds, decided to create the universe and place the human race on planet earth. They knew in foreknowledge that the human race would fall in the man Adam, and in that predeterminate counsel they knew one person of the triune God would have to be a Redeemer. Everything moved according to that predeterminate counsel of Acts chapter 2. As part of that same big picture, Les says God put His finger on every one of us and said, “I have a place waiting for you,” and we are in that predetermined position put there by the counsel of the triune God before anything was ever created.