Since eternal happiness, consisting in the vision of God, exceeds the common state of nature, and especially in so far as this is deprived of grace through the corruption of original sin, those who are saved are in the minority. In this especially, however, appears the mercy of God, that He has chosen some for that salvation, from which very many in accordance with the common course and tendency of nature fall short.
He seems to be relying on some sort of argument about proportion in difficulty. Eternal life, he says, is so much above us, wounded by mortal sin, that we cannot expect many to get there. Very well. But mortal sin is so much below our nature in itself (an unwounded human nature) that God has great mercy. And it is God in whom good begins, and good is greater than evil. So the argument really goes nowhere. It’s a good effort to understand the depth of the word “many are called but few are chosen” (Mt 22:14); but I don’t think it’s leading us down the right path.
The real lesson that I’d take from Jesus’ words “many are called but few are chosen” is that, if many are saved, sadly not many reach the transforming union before death; they attain only a relative perfection in avoiding mortal sin, perhaps only in their very last moments by the grace of God, the infinite merits of Christ, and the prayers of the Church, echoing God’s own desire, that none may die apart from Christ; and this relative perfection (the avoidance of mortal sin) is obtained only late and poorly rather than becoming transformed more extensively and more deeply into Christ before death. It would, then, be the prayers of the “few” that sustain the “many”, which is, simply how the Church has always understood to “co-redemptive” value of the saints’ presence on earth.
I’m not sure if this is the case. But it’s certainly a more logical reading than Thomas’ in my opinion.