Matthew 5:8

Blessed are the pure in heart: for they shall see God--Here, too, we are on Old Testament ground. There the difference between outward and inward purity, and the acceptableness of the latter only in the sight of God, are everywhere taught. Nor is the "vision of God" strange to the Old Testament; and though it was an understood thing that this was not possible in the present life (Exo 33:20; and compare Job 19:26-27; Isa 6:5), yet spiritually it was known and felt to be the privilege of the saints even here (Gen 5:24; Gen 6:9; Gen 17:1; Gen 48:15; Psa 27:4; Psa 36:9; Psa 63:2; Isa 38:3, Isa 38:11, &c.). But oh, with what grand simplicity, brevity, and power is this great fundamental truth here expressed! And in what striking contrast would such teaching appear to that which was then current, in which exclusive attention was paid to ceremonial purification and external morality! This heart purity begins in a "heart sprinkled from an evil conscience," or a "conscience purged from dead works" (Heb 10:22; Heb 9:14; and see Act 15:9); and this also is taught in the Old Testament (Psa 32:1-2; compare Rom 4:5-8; Isa 6:5-8). The conscience thus purged--the heart thus sprinkled--there is light within wherewith to see God.

— Jamieson-Fausset-Brown Commentary (public domain)