5And he who was seated on the throne said, “Behold, I am making all things new.” Also he said, “Write this down, for these words are trustworthy and true.”
In the flow of the passage the logical answer needs to be that the same speaker of the previous sentence gives the command of the present sentence. That is, it is God himself. That understanding adds to the urgency of obedience to the command.
5 And he who was seated on the throne said, “Behold, I am making all things new.” Also he said, “Write this down, for these words are trustworthy and true.”