Surely, all would agree that the expansion of the gospel is clearly seen in the New Testament. But, gentile conversions are not germane only to the New Testament. In fact, in Nineveh alone, the scriptures tell us that 120,000 people came to faith. Many people like to believe the expansion of the gospel, distinct as only a New Testament Principle, which it is not. There is an expansion, but it is not solely in the NT.
We see these gentile conversions in the Old Testament:
Noah, his wife and children
Zipporah (Possibly, her father, Jethro)
‘Mixed Multitude’ ex 12:38
Slaves that agreed to circumcision Gen 17:10-14
‘Many people of the Land became Jews’ Esther 8:17 *There is no way to actually tell which of these people were doing this out of fear alone….the scriptures tell us they were fearful ‘of the Jews’.
Nineveh-120k Jonah 3:5 *This is most likely the largest mass conversion documented. Nothing like this even in the NT.
Isaiah 56:6-8 ‘foreigners who bind themselves to the Lord…’
Exodus 12:48-49 “And when a stranger shall sojourn with thee…”
1st Kings 8:38 “What prayer and supplication soever be made by any man…”
Isaiah 2:3 “And many people shall go and say, Come ye, and let us go up to the mountain of the LORD…”
Num 32:12 Save Caleb the son of Jephunneh the Kenezite, and Joshua the son of Nun: for they have wholly followed the LORD.
Jos 15:17 And Othniel the son of Kenaz, the brother of Caleb, took it: and he gave him Achsah his daughter to wife.
Job and his three friends. Elihu, Job’s true and faithful comforter. Hiram, King of Tyre, the friend of David (whose mother was of the tribe of Dan).
Uriah the Hittite. Ahithophel and his family were also Hittites Araunah the Jebusite.
Joseph’s Egyptian steward Gen 43:23
The Queen of Sheba 1 Kings 10:8-9