The same way that they are of use today. The writers of the Gospels, including Paul, used the Old Testament -- all they had then -- to teach the Gentiles.
But the anointing which you have received from Him abides in you, and you do not need that anyone teach you; but as the same anointing teaches you concerning all things, and is true, and is not a lie, and just as it has taught you, you will abide in Him. 1 John 2:27
All the Gentiles did not have bibles.
Nor did Paul and the Apostles.
Pauls letters became scripture, but this came over the years, still the Church flourished.
JLB