Matthew 5:8

The "pure in heart" are those who are single-minded in their devotion to God and therefore morally pure inwardly. Inner moral purity is an important theme in Matthew and in the Old Testament (cf. Deuteronomy 10:16; Deuteronomy 30:6; 1 Samuel 15:22; Psalms 24:3-4; Psalms 51:6; Psalms 51:10; Isaiah 1:10-17; Jeremiah 4:4; Jeremiah 7:3-7; Jeremiah 9:25-26). Likewise freedom from hypocrisy is also prominent (cf. Psalms 24:4; Psalms 51:4-17; Proverbs 22:11; Matthew 6:22; Matthew 6:33). Jesus probably implied both ideas here.

The pure in heart can look forward to seeing God in the person of Messiah when He reigns on the earth (Psalms 24:3-4; Isaiah 33:17; Isaiah 35:2; Isaiah 40:5). Messiah would be single-minded in His devotion to God and morally pure. Thus there will be a correspondence and fellowship between the King and those of His subjects who share His character. No one has seen God in His pure essence without some type of filter. The body of Jesus was such a filter. Seeing God is a synonym for having intimate knowledge of and acquaintance with Him (John 14; 1 John 1:1-4).