“The Holy City is not the Church of this country only, but of the whole world as well: not that of this age only, but from Abel himself down to those who shall to the end be born and believe in Christ, the whole assembly of the Saints, belonging to one city; which city is Christ’s body, of which Christ is the Head. There, too, dwell the Angels, who are our fellow-citizens: we toil, because we are as yet pilgrims: while they within that city are awaiting our arrival. Letters have reached us too from that city, apart from which we are wandering: those letters are the Scriptures, which exhort us to live well.” -Augustine, Exposition of Psalm 91
BTW NOTES: Many places on the internet simply offer the paraphrase “Scriptures are our letters from home.” However, the above is the direct quote. The paraphrase is close, but it’s not exactly what Augustine said (as far as I can tell).