The argument begins in the previous verses by noting that those who deny Christ also deny the record of the gifts of the Spirit as recorded in the scriptures. A reason they might do so is that they might believe that although perhaps in ancient days such things might have happened, they do not happen in the current day.
That is the proposition against which he argues that they must continue, for “God is the same yesterday, today, and forever.” A definition of God is his trustworthiness and stability. Without that, He would not be God.
In contrast to the God that the unbelievers have imaged, Moroni proposes to “show unto you a God of miracles, even the God of Abraham, and the God of Isaac, and the God of Jacob.” The repetition of the “God of” the different patriarchs is meant to demonstrate that they all believed in the same God, and even though they lived at different times, God was to them, and for them, the same God.