Our Lord is often referred to as a shepherd. This particular parable alludes to the messianic prophecy of Ezekiel (34:11-31) in which God Himself would come down and shepherd His people, seeking them out and rescuing them:
Behold, I, I myself will search for my sheep, and will seek them out. As a shepherd seeks out his flock when some of his sheep have been scattered abroad, so will I seek out my sheep; and I will rescue them from all places where they have been scattered…
The parable is understood allegorically by St Anselm and St Hilary of Poitiers (both Fathers of the Church). The lost sheep represent mankind who had strayed through sin. The 99 sheep on the hills are the angels of God in heaven. In the Incarnation, God the Son descended ‘from the hills’ (i.e. from heaven) to seek the lost souls of men and to rescue them through His death and resurrection. And so Christ restores men to grace and leads them back to the company of angels in heaven (see Heb 12:22).
This shepherding of lost souls to safe pastures continues in the Church, for Christ continues to operate through the Church, His mystical Body. As the parable reminds us, “it is not the will of my Father who is heaven that one of these little ones should perish (v.14)”. So too in the Church, we must never cease to reach out to the lost, and be ready to shepherd them and provide them with the grace won by Christ in our redemption. As we await our coming Saviour, we have been given the gift of time in order that the lost may be found.