What God did through Jesus Christ, He had promised long beforehand. Consider how Paul refers to this fact in the opening of his letter to Titus:
Titus 1:1 Paul, a bond-servant of God and an apostle of Jesus Christ, for the faith of those chosen of God and the knowledge of the truth which is according to godliness,
Titus 1:2 in the hope of eternal life, which God, who cannot lie, promised long ages ago,
Titus 1:3 but at the proper time manifested, even His word, in the proclamation with which I was entrusted according to the commandment of God our Savior,
Titus 1:4 To Titus, my true child in a common faith: Grace and peace from God the Father and Christ Jesus our Savior.
This is the way of God – to announce His intentions to His prophets. This is so that His prophets might proclaim His intentions – that is, His promises – to His people. Those promises would be written down, and then once they were fulfilled, all the generations that followed could glorify God for His faithfulness to His word.
Blessed be God, who revealed to His prophets what He was going to do. As Amos said:
Amos 3:7 Surely the Lord GOD does nothing
Unless He reveals His secret counsel
To His servants the prophets.
Scripture quotations taken from the NASB, unless otherwise noted.