Louis Berkhof
“The true Israelites in the Old Testament, as well as in the New, are not the natural descendants of Abraham as such, but only they who share his faith. In the election of Israel, God did not, in the last analysis, aim at the separation of Israel as a nation, but at the formation of a spiritual people, primarily gathered out of the chosen race, but also in part out of the surrounding nations. From the earliest times, proselytes were incorporated into Israel. Solomon, in his dedicatory prayer, did not forget the stranger who might come to worship in the temple (I Kings 8:41 ff.); and the prophets looked forward with joyful expectancy to the time when the Gentiles, too, would bring their treasures into the temple of the Lord.
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“[The law] was not purely an external rule; the pious Israelite had it written on the tablets of his heart (Ps. 37:31; 40:8). They were not saved in any other way than New Testament believers. They needed the same Mediator and the same Holy Spirit, and received the same blessings of the covenant of grace, though not so abundantly, nor in exactly the same manner” (Principles of Biblical Interpretation, pp. 135-136).
The Sovereign Logos 