Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 23

Chapter 23.—Of the Word of the Lord to Abraham, by Which It Was Promised to Him that His Posterity Should Be Multiplied According to the Multitude of the Stars; On Believing Which He Was Declared Justified While Yet in Uncircumcision.

The word of the Lord came to Abraham in a vision also.  For when God promised him protection and exceeding great reward, he, being solicitous about posterity, said that a certain Eliezer of Damascus, born in his house, would be his heir.  Immediately he was promised an heir, not that house-born servant, but one who was to come forth of Abraham himself; and again a seed innumerable, not as the dust of the earth, but as the stars of heaven,—which rather seems to me a promise of a posterity exalted in celestial felicity.  For, so far as multitude is concerned, what are the stars of heaven to the dust of the earth, unless one should say the comparison is like inasmuch as the stars also cannot be numbered?  For it is not to be believed that all of them can be seen.  For the more keenly one observes them, the more does he see.  So that it is to be supposed some remain concealed from the keenest observers, to say nothing of those stars which are said to rise and set in another part of the world most remote from us.  Finally, the authority of this book condemns those like Aratus or Eudoxus, or any others who boast that they have found out and written down the complete number of the stars.  Here, indeed, is set down that sentence which the apostle quotes in order to commend the grace of God, “Abraham believed God, and it was counted to him for righteousness;” 909 lest the circumcision should glory, and be unwilling to receive the uncircumcised nations to the faith of Christ.  For at the time when he believed, and his faith was counted to him for righteousness, Abraham had not yet been circumcised.

Footnotes

Rom. 4:3, Gen. 15:6.