From the Holy Gospel according to Matthew
Matt 13:44
At that time Jesus spake unto His disciples this parable The kingdom of heaven is like unto treasure hid in a field. And so on.

Homily by Pope St Gregory the Great
11th on the Gospels
Dearly beloved brethren, the kingdom of heaven is likened unto the things of earth, to the end that by the mean of things which we know, our mind may rise to the contemplation of the things which we know not by the example of things which are seen, may fix her gaze on things which are not seen by the touch of things which she useth, may be warmed towards the things which she useth not; by things which she knoweth and loveth, to love also the things which she knoweth not. For, behold, "the kingdom of heaven is likened unto treasure hid in a field, the which when a man hath found, he hideth, and, for joy thereof, goeth and selleth all that he hath and buyeth that field."

And herein we must remark that the treasure, when once it hath been found, is hidden to keep it safe. He whose intimate yearnings after God are not hidden from the praise of men, is open thereby to the attacks of evil spirits. In this life we are, as it were, journeying homewards on a road beset by evil spirits who are like highwaymen. He therefore inviteth robbery who carrieth his treasure ostentatiously. Doubtless our neighbour should be able to see our good works, as it is written: "Let your light so shine before men that they may see your good works, and glorify your Father which is in heaven." But this is not to be understood to mean that we are to seek the praise of men by what we do. Rather, let us in such wise work in the open that the inner intention of devotion is not advertised. So we shall give an example to our neighbour, and yet keep hidden, except from the sight of God, our purpose of pleasing him.

The treasure is the desire for heaven, the field wherein it is hidden is the earnest observance wherewith this desire is surrounded. Whosoever turneth his back upon the enjoyments of the flesh, and by earnest striving heavenward, putteth all earthly lusts under the feet of discipline, so that he smileth back no more when the flesh smileth at him, and shuddereth no more at anything that can only kill the body whosoever doth thus, hath sold all that he had, and bought that field.