Now this kingdom of God some think to be higher and better than the heavenly kingdom, but some think it to be one and the same in reality, but called by different names; at one time the kingdom of God from Him who reigneth, but at another the kingdom of heaven from the Angels and Saints, His subjects, who are said to be of heaven.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: lib. iii. ep. 206.