John Chrysostom

Blessed are the pure in heart, for they shall see God. Behold again the reward is spiritual. Now He here calls pure, either those who have attained unto all virtue, and are not conscious to themselves of any evil; or those who live in temperance. For there is nothing which we need so much in order to see God, as this last virtue. Wherefore Paul also said, Follow peace with all men, and holiness, without which no man shall see the Lord. He is here speaking of such sight as it is possible for man to have. …

Homily on the Gospel of Matthew 15 ↗

Theophylact of Ohrid

Blessed are the pure in heart, for they shall see God. There are many who are not rapacious and greedy, but are generous in almsgiving, yet they fornicate and commit other uncleanliness. Christ commands, therefore, that along with the other virtues we should also be pure, that is, chaste and temperate, not only in the body, but in the heart as well. Without holiness, namely, chastity, no one will see the Lord. …

Commentary on Matthew ↗

Augustine of Hippo

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 ye Him; (Wisd. 1:1.) the single heart is the same as is here called the pure heart. (Civ. Dei, xxii 29.) But if spiritual eyes in the spiritual body shall be able only to see so much as they we now have can see, undoubtedly God will not be able to be seen of them. (de Trin. i. …

Catena Aurea by Aquinas ↗

Ambrose of Milan

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; the next that follows therefore is, Blessed are the pure in heart.

Catena Aurea by Aquinas ↗

Jerome

Blessed are the pure in heart, for they will see God. Those who have no consciousness of sin are not accused. The world is seen by the pure in heart: the temple of God cannot be defiled.

Commentary on Matthew ↗