“Blessed are the pure in heart: for they shall see God” (Mt 5:8).
“The merciful loses the benefit of his mercy, unless he shows it from a pure heart; for if he seeks to have whereof to boast, he loses the fruit of his deeds.” -St. Ambrose
“Purity of heart comes properly in the sixth place, because on the sixth day man was created in the image of God, which image was shrouded by sin, but is formed anew in pure hearts by grace. It follows rightly the before-mentioned graces, because if they be not there, a clean heart is not created in a man.” -Glossa apud Anselm
“By the pure are here meant those who possess a perfect goodness, conscious to themselves of no evil thoughts, or again those who live in such temperance as is mostly necessary to seeing God according to that of St. Paul, ‘Follow peace with all men, and holiness, without which no man shall see God.’ For as there are many merciful, yet unchaste, to show that mercy alone is not enough, he adds this concerning purity.” -St. John Chrysostom
“The pure is known by purity of heart, for the temple of God cannot be impure.” -St. Jerome
“He who in thought and deed fulfils all righteousness, sees God in his heart, for righteousness is an image of God, for God is righteousness. . . . In that world to come the pure in heart shall see God face to face, not in a glass, and in enigma as here.” -Pseudo-Chrysostom
“They are foolish, who seek to see God with the bodily eye, seeing He is seen only by the heart, as it is elsewhere written, ‘In singleness of heart seek Him’ (Wis 1:1); the single heart is the same as is here called the pure heart.” -St. Augustine
“Seeing God is the reward of faith; to which end our hearts are made pure by faith, as it is written, ‘cleansing their hearts by faith’ (Acts 15:9).” -St. Augustine
“No one seeing God can be alive with the life men have on earth, or with these our bodily senses. Unless one die altogether out of this life, either by totally departing from the body, or so alienated from carnal lusts that he may truly say with the Apostle, ‘whether in the body or out of the body, I cannot tell,’ he is not translated that he should see this vision.” -St. Augustine
Quotations from St. Thomas Aquinas, Catena Aurea: Commentary on the Four Gospels Collected Out of the Works of the Fathers, Vol. I, Part I (Oxford: John Henry Parker, 1841).