14 Was, then, the hair of his head of such importance that, so long as it remained, his strength should endure unconquered, but when his head was shorn the man should suddenly lose all his strength? It is not so, nor may we think that the hair of his head has such power. There is the hair of religion and faith; the hair of the Nazarite perfect in the Law, consecrated in sparingness and abstinence, with which she (a type of the Church), who poured ointment on the feet of the Lord, wiped the feet of the heavenly Word, for then she knew Christ also after the flesh. That hair it is of which it is said: “Your hair is as flocks of goats,” growing on that head of which it is said: “The head of the man is Christ,” and in another place: “His head is as fine gold, and his locks like black pine-trees.”
15. And so, also, in the Gospel our Lord, pointing out that some hairs are seen and known, says: “But even the hairs of your head are all numbered,” implying, indeed, acts of spiritual virtues, for God does not take care for our hair. Though, indeed, it is not absurd to believe that literally, seeing that according to His divine Majesty nothing can be hidden from Him.
16. But what does it profit me, if God Himself knows all my hairs? That rather abounds and profits me, if the watchful witness of good works reward me with the gift of eternal life. And, in fine, Samson himself, declaring that these hairs are not mystical, says: “If I be shorn my strength will depart from me.” So much concerning the mystery, let us now consider the order of the passage.
Source: On the Holy Spirit (New Advent)