score:6
The reference is probably to Augustine's Sermon 61, on Luke 13:21–24 – in which Jesus compares the kingdom of God to leaven in bread, and references the narrow gate. Augustine says:
“The three measures of meal” of which the Lord spake, is the human race. Recollect the deluge; three only remained, from whom the rest were to be re-peopled. Noe had three sons, by them was repaired the human race. That holy “woman who hid the leaven,” is Wisdom. Lo, the whole world crieth out in the Church of God, “I know that the Lord is great.” Yet doubtless there are but few who are saved.
Ye remember a question which was lately set before us out of the Gospel, “Lord,” it was said, “are there few that be saved?” What said the Lord to this? He did not say, “Not few, but many are they who are saved.” He did not say this. But what said He, when He had heard, “Are there few that be saved? Strive to enter by the strait gate.” When thou hearest then, “Are there few that be saved?” the Lord confirmed what He heard.
So Augustine understands the Lord's words in Luke 13:24 as answering the question in the affirmative – yes, there are few that will be saved. So Augustine's certainty – "doubtless there are few who are saved" – is based on his interpretation of the words of Jesus in this passage.