‘The gospel of God, (which he had promised to fall beforehand by his prophets in the Holy Scriptures,)’ Our translators have put verse 2 in brackets as a parenthesis. Grammatically that is right, but it is still very, very important.
If you're a youngster, and you've had your head filled with the propaganda of atheistic society: everything can be explained materially. There's only the material; there’s no spiritual. Your teachers, the people who bombard you with this, have never considered Scriptural prophecy, the astonishing, miraculous nature of it. They are discarding is and brushing is aside. But this is the only message in the world that has ever been prophesied, promised, detailed beforehand, so that there can be no mistake, no delusion, no falsehood.
God does not make things up as he goes along. God always intended that Christ should come and be our deliverer. This has always been the way of salvation, and these things were published in advance, so that we can appreciate the divine plan.