Theodoret of Cyrus: City of God is a Way of Life

But as it is, they hanker after something better, namely, heaven; hence God is not ashamed to be called their God: he prepared a city for them (v.16). He refers not to some building but to the way of life in heaven; in fact, he says as much a little later, But you have come to Mount Sion, city of the living God, heavenly Jerusalem.

- Theodoret of Cyrus (around A.D. 393 to around A.D. 457), Commentary on Hebrews, Chapter 11, in Theodoret of Cyrus, Commentary on the Letters of St. Paul, Volume 2, p. 184 (2001), Robert C. Hill translator.