Luke 15:7

Now the angels, inasmuch as they are intelligent beings, do not unreasonably rejoice at the redemption of men, as it follows, I say unto you, that likewise joy shall be in heaven over one sinner that repenteth, more than over ninety and nine just persons who need no repentance. Let this serve as an incentive to goodness, for a man to believe that his conversion will be pleasing to the assembled angels, whose favour he ought to court, or whose displeasure to fear.

(ubi sup.) But he allows there is more joy in heaven over the converted sinner, than over the just who remain stedfast; for the latter for the most part, not feeling themselves oppressed by the weight of their sins, stand indeed in the way of righteousness, but still do not anxiously sigh after the heavenly country, frequently being slow to perform good works, from their confidence in themselves that they have committed no grievous sins. But, on the other hand, sometimes those who remember certain iniquities that they have committed, being pricked to the heart, from their very grief grow inflamed towards the love of God; and because they consider they have wandered from God, make up for their former losses by the succeeding gains. Greater then is the joy in heaven, just as the leader in battle loves that soldier more who having turned from flight, bravely pursues the enemy, than him who never turned his back and never did a brave act. So the husbandman rather loves that land which after bearing thorns yields abundant fruit, than that which never had thorns, and never gave him a plentiful crop. But in the mean time we must be aware that there are very many just men in whose life there is so much joy, that no penitence of sinners however great can in any way be preferred to them. Whence we may gather what great joy it causes to God when the just man humbly mourns, if it produces joy in heaven when the unrighteous by his repentance condemns the evil that he has done.

We may hence understand the extent of our Saviour's kingdom. For He says there are a hundred sheep, bringing to a perfect sum the number of rational creatures subject to Him. For the number hundred is perfect, being composed of ten decades. But out of these one has wandered, namely, the race of man which inhabits earth. But was He then angry with the rest, and moved by kindness only to one? By no means. For they are in safety, the right hand of the Most Mighty being their defence. It behoved Him rather to pity the perishing, that the remaining number might not seem imperfect. For the one being brought back, the hundred regains its own proper form.