O death, where is thy sting? O grave, where is thy victory?
Acts 26:18 to open their eyes, in order to turn them from darkness to light, and from the power of Satan to God, that they may receive forgiveness of sins and an inheritance among those who are sanctified by faith in Me.’
Here we find Paul's commission to the gentiles included that they may receive forgiveness of sins.
We know that only those in Adam had the sin and needed the death to be defeated. Why then did gentiles need forgiveness of sin? What is the connection between covenant sin and the sins of the gentiles (if any)? Can there be sin apart from covenant relationship with God?
The Gentiles that you are referring to in verse 17 DID have a Covenant with YHWH. Read the whole chapter, focus on verses 6 and 7. The Apostle Paul was preaching Nothing But THE HOPE of THE FATHERS, of THE PROMISE that was given to the Twelve Tribes. (What is this 'HOPE/PROMISE', you may ask, that Paul is referring to? Redemption, Resurrection, Salvation)
You see, all twelve tribes were 'Married' to YHWH, But YHWH gave The House of Israel a 'Bill of Divorce' because of her transgressions (Jer. 3:8). (Amo 9:9) (Hos 8:8) (Mic 5:8) and sifted the House of Israel among the Gentile nations and they Became 'as' Gentiles. But the New Covenant Hope/Promises could only be Fulfilled with/for, the Whole House of Israel. Remnants (12,000) from all twelve tribes. Paul went to those of the Diaspora with the Gospel, unto the New Covenant for the Old Covenant Peoples.