Albert Barnes on Matthew 5:8

Blessed are the pure in heart. That is, whose minds, motives, and principles are pure. Who seek not only to have the external actions correct, but who desire to be holy in heart, and who are so. Man looks on the outward appearance, but God looketh on the heart.

They shall see God. There is a sense in which all shall see God, Rev 1:7. That is, they shall behold him as a Judge, not as a Friend. In this place it is spoken of as a peculiar favour. So also in Rev 22:4. And they shall see his face. To see the face of one, or to be in his presence, were, among the Jews, terms expressive of great favour. It was regarded as a high honour to be in the presence of kings and princes, and to be permitted to see them, Prov 22:29. He shall stand before kings, etc. See also 2Ki 25:19. "Those that stood in the king's presence ;" in the Hebrew, those that saw the face of the king; that is, who were his favourites and friends. So here, to see God, means to be his friends and favourites, and to dwell with him in his kingdom.

(d) "pure in heart" Ps 24:3,4, He 12:4, 1Jn 3:2,3