The following is an excerpt from Logos Bible software’s commentary on 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 (Ex. 33:20; and compare Job 19:26, 27, IS 6:5), yet spiritually it was know and felt to be the privilege of the saints even here (Ge. 5:24; 6:9; 17:1; 48:15; Ps 27:4; 36:9; 63:2; Is 38:3, 11 & etc). 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; 9:14; and see Ac 15:9); and this also is taught in the Old Testament (Ps 32:1, 2; compare Ro 4:5-8; Is 6:5-8). The conscience thus purged—the heart thus sprinkled—there is light within wherewith to see God. “If we say that we have fellowship with Him, and walk in darkness, we lie, and do not the truth; but if we walk in the light, as He is in the light, we have fellowship with the other”—He with us and we with Him—”and the blood of Jesus Christ His Son cleanseth us”—us who have this fellowship, and who,without such continual cleansing, would soon lose it again—”from all sin” (1 Jn 1:6,7). “Whosoever sinneth hath not seen Him, neither known Him” (1 Jn 3:6); “He that doeth evil hath not seen God” (3 Jn 1:11). The inward vision thus clarified, and the whole inner man in sympathy with God, each looks upon the other with complacency and joy, and we are “changed into the same image from glory to glory.” But the full and beatific vision of God is reserved for that time to which the Psalmist stretches his views—”As for me, I shall behold Thy face in righteousness: I shall be satisfied, when I awake, with Thy likeness” (Ps 17:15). Then shall His servants serve Him: and they shall see His face; and His name shall be in their foreheads (Rev 22:3, 4). They shall see Him as He is (1 Jn 3:2). But, says the apostle, expressing the converse of this beatitude—”Follow holiness, without which no man shall see the Lord” (Heb 12:14).