The heart has only one opening. Obviously, that is a horribly inaccurate statement about the human anatomy. But it is an important spiritual truth. Spiritually, the human heart has only one opening: everything we both give and receive in the spiritual life passes through it. Thus, to the extent we widen our hearts' abitlity to give, we widen them also to receive. And vice versa. Likewise, to the extent we narrow our hearts' ability to give — by hatred, envy, lust, or greed — so we narrow our capacity to receive. In short, the more we render our hearts capable to give love, the more capable they are to receive love; and the more we harden our hearts by refusing love, so we harden them against love.
So our Lord says, "Stop judging and you will not be judged. Stop condemning and you will not be condemned. Forgive and you will be forgiven" (Lk 6:37). On the surface this is just good, sound advice. In fairness we should not expect to receive ourselves what we refuse to extend to others. Yet we should not imagine that God gauges what we give and then gives us precisely the same, or that He begrudgingly withholds His forgiveness until we have given ours. There is a deeper truth at play here. By a strict interior logic of the soul, the way we dispose our hearts to others determines our disposition to God as well. If we narrow or harden our hearts by judging and condemning, then the mercy of God will find our hearts closed and impenetrable. If, however, we widen our hearts in forgiveness, then God's forgiveness likewise finds them open.
This truth of the heart draws attention also to the unity of the two commandments, to love God and to love our neighbor (cf. Mt 22:37-40). We cannot narrow our heart to our neighbor and think it remains wide open to God. Narrowness of heart to our neighbor means also and necessarily narrowness of heart to God. This helps explain also that difficult petition of the Our Father — "Forgive us our trespasses as we forgive those who trespass against us" (Mt 6:12) — as well as our Lord's commentary on that petition (cf. Mt 6:14-15). Again, it is not that He refuses to extend His forgiveness. Rather, by our own lack of forgiveness we make our hearts incapable of receiving His. And with that petition we bow before that truth.
Further, the one opening of the heart determines our reception of grace. God is not stingy. He gives His grace abundantly: "a good measure, packed together, shaken down, and overflowing, will be poured into your lap" (Lk 6:38). We, however, are stingy in our openness. By our lack of charity we provide Him a very small opening to work with. We narrow our hearts to receive His grace and then wonder why His grace has been ineffective. His Sacred Heart remains pierced — open — to us. Yet our hearts remain narrowed — or closed entirely — to Him.
Thus, there is the need to prepare ourselves for the reception of the Sacraments, especially Confession and Holy Communion. The Sacraments always communicate God's grace. But that grace often finds a narrow opening to our hearts and consequently achieves very little within us. If we want His grace to accomplish His will and establish His peace within us, we must dispose ourselves to receive it. "Give and gifts will be given to you" (Lk 6:38). Time spent widening our hearts in reflection, prayer, and meditation before entering the Confessional or getting in the Communion line makes an enormous difference in the fruitfulness of the grace we receive. "For the measure you measure out will be measured back to you" (Lk 6:38).
Of course, the final word ought not be a strict accounting or a precise measuring. Rather, it should be generosity. And God will not be outdone in generosity. Thus the measure with which we ought to love God, as St. Bernard says, is to love God without measure.