48 Now Simeon the priest, when he took Him up in his arms to present Him before God, understood as he saw [Him] that He was not presenting Him, but was being himself presented. For the Son was not presented by the servant to His Father, but the servant was presented by the Son to his Lord. For it is not possible that He, by Whom every offering is presented, should be presented by another. For the offering does not present him that offers it; but by them that offer are offerings presented.
So then He Who receives offerings gave Himself to be offered by another, that those who presented Him, while offering Him, might themselves be presented by Him. For as He gave His body to be eaten, that when eaten It might quicken to life them that ate Him; so He gave Himself to be offered, that by His Cross the hands of them that offered Him might be sanctified. So, then, though the arms of Simeon seemed to be presenting the Son, yet the words of Simeon testified that he was presented by the Son.
Therefore we can have no dispute concerning this, because that which was said put an end to dispute— Now let Your servant depart in peace. He then who is let depart to go in peace to God, is presented as an offering to God. And in order to make known by whom he was presented, he said— For lo! My eyes have seen Your mercy. If there was no grace wrought on him, why then did he give thanks? But rightly did he give thanks, that he was thought worthy to receive in his arms Him, Whom angels and prophets greatly desired to see.
For lo! my eyes have seen Your mercy. Let us understand then and see. Is mercy that which shows mercy to another, or is it that which receives mercy from another? But if mercy is that which shows mercy to all, well did Simeon call our Lord by the name of the mercy that showed mercy to him—Him Who freed him from the world which is full of snares, that he might go to Eden which is full of pleasures; for he who was priest said and testified that he was offered as an offering, that from the midst of the perishing world he should go and be stored up in the treasure-house which is kept safe.
For one for whom it may be that what he has found should be lost, to him it belongs to be diligent that it should be kept safe. But for our Lord it could not be that He should be lost; but by Him the lost were found. So then, through the Son Who could not be lost, the servant who was very desirous not to be lost, was presented. Lo! My eyes have seen Your mercy. It is evident Simeon received grace from that Child Whom he was carrying. For inwardly he received grace from that Infant, Whom openly he received in his arms. For through Him Who was glorious, even when He was carried, being small and feeble, he that carried Him was made great.
Source: On Our Lord (New Advent)