71 Since, therefore, a command had been given that what is holy should not be given to dogs, and pearls should not be cast before swine, a hearer might object and say, conscious of his own ignorance and weakness, and hearing a command addressed to him, that he should not give what he felt that he himself had not yet received—might (I say) object and say, What holy thing do you forbid me to give to the dogs, and what pearls do you forbid me to cast before swine, while as yet I do not see that I possess such things? Most opportunely He has added the statement: “Ask, and it shall be given you; seek, and you shall find; knock, and it shall be opened unto you. For every one that asks receives; and he that seeks finds; and to him that knocks it shall be opened.” The asking refers to the obtaining by request soundness and strength of mind, so that we may be able to discharge those duties which are commanded; the seeking, on the other hand, refers to the finding of the truth. For inasmuch as the blessed life is summed up in action and knowledge, action wishes for itself a supply of strength, contemplation desiderates that matters should be made clear: of these therefore the first is to be asked, the second is to be sought; so that the one may be given, the other found. But knowledge in this life belongs rather to the way than to the possession itself: but whoever has found the true way, will arrive at the possession itself which, however, is opened to him that knocks.
72. In order, therefore, that these three things— viz. asking, seeking, knocking— may be made clear, let us suppose, for example, the case of one weak in his limbs, who cannot walk: in the first place, he is to be healed and strengthened so as to be able to walk; and to this refers the expression He has used, “Ask.” But what advantage is it that he is now able to walk, or even run, if he should go astray by devious paths? A second thing therefore is, that he should find the road that leads to the place at which he wishes to arrive; and when he has kept that road, and arrived at the very place where he wishes to dwell, if he find it closed, it will be of no use either that he has been able to walk, or that he has walked and arrived, unless it be opened to him; to this, therefore, the expression refers which has been used, “Knock.”
73. Moreover, great hope has been given, and is given, by Him who does not deceive when He promises: for He says, “Every one that asks, receives; and he that seeks, finds; and to him that knocks, it shall be opened.” Hence there is need of perseverance, in order that we may receive what we ask, and find what we seek, and that what we knock at may be opened. Now, just as He talked of the fowls of heaven and of the lilies of the field, that we might not despair of food and clothing being provided for us, so that our hopes might rise from lesser things to greater; so also in this passage, “Or what man is there of you,” says He, “whom if his son ask bread, will he give him a stone? Or if he ask a fish, will he give him a serpent? If you then, being evil, know how to give good gifts unto your children, how much more shall your Father which is in heaven give good things to them that ask Him?” How do the evil give good things? Now, He has called those evil who are as yet the lovers of this world and sinners. And, in fact, the good things are to be called good according to their feeling, because they reckon these to be good things. Although in the nature of things also such things are good, but temporal, and pertaining to this feeble life: and whoever that is evil gives them, does not give of his own; for the earth is the Lord's, and the fullness thereof, who made heaven, and earth, the sea, and all that therein is. How much reason, therefore, there is for the hope that God will give us good things when we ask Him, and that we cannot be deceived, so that we should get one thing instead of another, when we ask Him; since we even, although we are evil, know how to give that for which we are asked? For we do not deceive our children; and whatever good things we give are not given of our own, but of what is His.
Source: Our Lord's Sermon on the Mount (New Advent)