Question posited in a Jewish religious forum:

“Christians, how do you explain the book of Jonah and the people of Nineveh (gentiles) repenting and being saved prior to Jesus?”

Christian response,

From Adam forward, humanity was provided covenant righteousness with Elohim by faith-trust in Elohim as the One true Deity, our Creator and Sustainer; this same truism was relevant for those in Nineveh.

Messiah’s presence in Time and His work at Golgotha simply paves the path to life in Eternity in response to the prerequisite of Leviticus 17:11 which was first pictured for humanity in Genesis 3:21; that is, the shedding of innocent blood as atonement for sin. It is “faith” in Jesus as Messiah that initiates a covenant of righteousness with the Father and it is the blood of Jesus that permits entry into the Kingdom via satisfactory atonement as mandated by the Father (1 Peter 1:18-19).

Though men and women in the Old Testament found righteousness with Elohim via “faith” (Hebrews 11), their sin required atonement before entering the Kingdom, a Kingdom where “nothing impure is permitted to enter therein” (Revelation 21:27); therefore, the Old Testament saints were interned in Sheol/paradise (Hades in the Greek) awaiting Messiah’s work at Golgotha.

Subsequent Messiah’s atoning sacrifice, those who die in body having expressed faith in Messiah Jesus are now immediately translated into the presence of Elohim in a disembodied spiritual state awaiting the resurrection as modeled by Messiah (2 Corinthians 5:8; Philippians 3:21). The imperishable resurrected body will be the eternal life form for all of redeemed humanity throughout Eternity.

Finally, it’s important to note that the people of Nineveh repented of their sin in response to Jonah’s preaching and sought righteousness through faith exhibited, validated, by their righteous efforts to eschew evil; unfortunately, much like the people of Israel, the Ninevites did not hold fast to their covenant of repentance and obedience but once again fell prey to defilement and were subsequently judged harshly for their unfaithfulness.