In the Lord’s Prayer, the petition for our “daily bread” is normally understood to signify all of our bodily needs and whatever we require to sustain our lives in this world. The Greek Patristic Fathers knew that the koine Greek word translated as “daily” is a unique term “ἐπιούσιον” (epiousion), which is only used in the New Testament Lord’s Prayer. This indicates that the word had special significance, as there were any number of other common Greek words to express the idea of “daily”. Epiousion literally means “needful”, “essential”, “super-substantial”, or “super-essential”. Understood in that sense, it takes on the more spiritual meaning of the nourishment of our souls by the Word of God, Jesus Christ who is the “Bread of Life;” the “Bread of God which has come down from heaven and given life to the world” (Jn 6.33–36); the bread which “a man may eat of it and not die,” but “live forever” (Jn 6.50–51). Thus the prayer for “daily bread” becomes the petition for daily spiritual nourishment through abiding communion with Christ so that one might live perpetually with God. 

Origen of Alexandria and Gregory of Nyssa are two examples of early (3^rd and 4^th century) Church Fathers who contributed significantly to the understanding of the unique word epiousion; both interpreting it as referring to the spiritual sustenance provided by God, emphasizing the need for divine support in daily life.

With that introduction, here is what St. Gregory of Nyssa (335 – 395) had to say about it:

From:  Ancient Christian Writers, No.18. Edited by Johannes Quasten and Joseph C. Plumpe. St. Gregory of Nyssa, The Lord’s Prayer – The Beatitudes, Trans. and annotated by Hilda C. Graef, 1954 Newman Press.  Pp. 68-70

Excerpt from:

SERMON 4  Thy Will be done, on earth as it is in Heaven. Give us this day our daily [ἐπιούσιον (epiousion)] bread.

Full of meaning is also the addition of this day [σήμερον (sémeron)], when He says: Give us this day our daily [ἐπιούσιον (epiousion)] bread. These words contain yet another teaching. For you should learn through what you say that the human life is but the life of a day. Only the present each one of us can call his own; the hope of the future is uncertain, for we know not what the day to come may bring forth. Why then do we make ourselves miserable worrying about the future? He says, Sufficient for the day is the evil thereof, evil here meaning the enduring of evil. Why are we disturbed about the morrow? By the very fact that He gives you the commandment for today, He forbids you to be solicitous for the morrow. He says to you as it were: He who gives you the day will give you also the things necessary for the day. Who causes the sun to rise? Who makes the darkness of the night disappear? Who shows you the rays of light? Who revolves the sky so that the source of light is above the earth? Does He who gives you so great things need your help to supply for the needs of your flesh? Do animals take care for their livelihood? Do ravens have tilled land or eagles barns? Is not the one means of providing a livelihood for all the Will of God, by which all things are governed? Therefore even an ox or an ass, or any other animal is taught its way of life by instinct, and it manages the present well but does not concern itself in the least with what comes afterwards. And should we need special advisers in order to understand that the life of the flesh is perishable and transitory? Are we not taught by the misfortunes of others, not chastened by those of our own life?

What profits this rich man his wealth? Like a fool he chases vain hopes, pulling down, building up, hoarding and dissipating, shutting up long periods of years as it were in barns, without letting them bear fruit. Will not one night prove false all these imagined hopes, like some vain dream about a nonentity? The life of the body belongs only to the present, but that which lies beyond us and is apprehended by hope belongs to the soul. Yet men in their folly are quite wrong about the use of either; they would extend their physical lives by hope, and draw the life of the soul towards enjoyment of the present. Therefore the soul is occupied by the world of sense and necessarily estranged from the subsisting reality of hope. What hope it has leans upon unstable things over which it has no control or authority.

Let us therefore learn from the counsel under consideration what one must ask for today, and what for later. Bread is for our use today; the Kingdom belongs to the beatitude for which we hope. By bread He means all our bodily requirements. If we ask for this, the man who prays will clearly understand that he is occupied with something transitory; but if we ask for something of the good things of the soul it will be clear that the petition concerns the everlasting realities, for which He commands us to be most concerned in our prayers. Thus the first necessity is put in its right place by the greater one. Seek ye, He says, the kingdom and justice, and all these things shall be added unto you; in Christ Jesus Our Lord, to whom be glory and power for ever and ever.   Amen.

Original Greek words used by Nyssen are in brackets [].  From: Gregorii Nysseni, De Oratione Dominica, De Beatitudinibus, Edidit Johannes F. Callahan, 1992 E.J. Brill. P. 56