The apostle Paul, writing in the late 40s to late 50s A.D., said, “Tell me, you who want to be under law, do you not listen to the law? For it is written that Abraham had two sons, one by the bondwoman and one by the free woman. But the son by the bondwoman was born according to the flesh, and the son by the freewoman through the promise. This is allegorically speaking (emphasis added), for these women are two covenants; one proceeding from Mount Sinai bearing children who are to be slaves; she is Hagar. Now, this Hagar is Mount Sinai in Arabia and corresponds to the current Jerusalem, for she is in slavery with her children. But the Jerusalem above is free; she is our mother” (Galatians 4:21-26).
“And you brethren, like Isaac, are children of promise. But as at that time, he who was born according to the flesh persecuted him who was born according to the spirit, so it is now also. But what does the Scripture say? ‘Cast out the bondwoman with her son, for the son of the bondwoman shall not be an heir with the son of the free woman’ (ref Genesis 21:10). So then brethren, we are not children of a bondwoman, but of a free woman” (Galatians 28-31).
E. W. Bullinger describes “Allegory” thus, “The Allegory is of two kinds, one in which it is continued metaphor, where the two things are both mentioned, and what is asserted belongs to the principal object. The other, in which it is continued hypocatastasis, where only one thing is mentioned, and what is asserted belongs properly to the secondary object.” Bullinger continues, “In Genesis, what is stated of Israel and Ishmael, Sarah and Hagar is all true history, yet in Galatians four, it is made to speak of and set forth other truths, and hence there it is and is called an allegory. At any rate, we have only one which is distinctly declared to be such (Galatians 4:22, 24).”
He goes on to say, “An allegory may sometimes be fictitious, but Galatians four shows us that a true history may be allegorized ( i.e., be shown to have further teaching in that which actually took place) without detracting from the truth of the history). Here note this important fact, that, in either case, Allegory is always stated in the past tense and never in the future. Allegory is thus distinguished from Prophecy. The Allegory brings other teaching out of past events, while the Prophecy tells us events that are yet to come, and means exactly what is said”  (all emphasis Bullinger).
Milton Terry, writing about the same Galatians passage, says, “The allegorizing process by which Paul makes Hagar and Sarah illustrate two covenants, is an exceptional New Testament instance of developing a mystical meaning from facts of Old Testament history.” He further says, “We observe that the apostle, first of all, states the historical facts, as written in the Book of Genesis, namely, that Abraham was the father of two sons, one by the bondwoman, the other by the freewoman. The son of the bondmaid was born according to the flesh (emphasis Terry), i.e., according to the ordinary course of nature, but the son of the freewoman was born through promise and, as the Scripture shows, by miraculous interjection” (cites Genesis 17:19; 18:10-14).
Terry continues, “He [Paul] further on brings in the Rabbinical tradition founded on Genesis 21:9, that Ishmael persecuted Isaac, perhaps having in mind also some subsequent aggressions of the Ishmaelites upon Israel, and then adds the words of Sarah, as written in Genesis 21:10, adapting them somewhat freely to his purpose. It is evident from all this that Paul recognizes the grammatico-historical truthfulness of the Old Testament narrative” (emphasis added). Continuing, he says, “But, he [Paul] says, all these historical facts are capable of being allegorized.” Then giving the Greek phrase, translates, “which things are allegorical” (emphasis Terry).
Furthermore, Terry says, “He [Paul] proceeds to allegorize the facts referred to, making the two women represent the two covenants, the Sinaitic (Jewish) and the Christian, and showing in detail how one thing answers to, or ranks with [emphasis Terry] another, and also wherein the two covenants stand opposed. The general import of the apostle’s language is clear and simple, and this allegorizing process served most aptly both to illustrate the relations and contrasts of the Law and the Gospel and also to confound and silence the Judaizing legalists, against whom Paul was writing.”
Then he says, “Here arises the important hermeneutical question, What inference are we to draw from this example of an inspired apostle allegorizing the facts of sacred history? Was it a fruit of his rabbinical education and a sanction of that allegorical method of interpretation, which was prevalent, especially among Jewish-Alexandrian writers, at that time?” 
Those are good questions indeed, which we will answer next time.
 Figures of Speech Used in the Bible, 1898 (748-749).
 Biblical Hermeneutics, 1883 (321-322)