Are you not aware that the Gospel itself is the power of God unto salvation, and that the Gospel itself if is the "seed" of the New Birth? If the unregenerate could not believe the Gospel, then Christ would not have commanded that it be preached to EVERY CREATURE.
I think that Matt 16:15 has been accomplished in Colossians 1:23.
Jesus instructed his apostles to go and preach to the "lost sheep"(God's regenerated children) of the house of Israel. The "house of Israel" is Jacob/Israel, Gen 33:28. All Israel is not of Israel,Rom 9:6. Jacob/Israel is representative of God's elect,Rom 9:11.
Eph 2 says that the natural man while he was yet spiritually dead (unable to understand spiritual things) God quickened him to a spiritual life.
I suppose that you have not checked the Greek meaning of Save, Saved and Salvation, which means, deliver, delivered and deliverance.
There is an eternal deliverance, and there are several deliverances that the regenerated person receives as he sojourns here in this world.
The gospel is, indeed, the power of God unto deliverance to every one that believeth.
There is a deliverance here on earth, when a newborn babe in Christ matures enough, by hearing the truths preached, to come unto a knowledge of the gospel.
The house of Israel consists of a portion of every kindred, and tongue, and person, and nation. Preachers of the true doctrine of Jesus Christ are told to go and preach to, regenerated people of the house of Israel, in every nation.