Answer: "Blessed are the pure in heart, for they will see God" (Matthew 5:8). Jesus spoke this during His famous Sermon on the Mount. So what did He mean by "pure in heart?"
The Greek word for "pure" in Matthew 5:8 is katharos. It means to be "clean, blameless, unstained from guilt." Interestingly, the word can refer specifically to that which is purified by fire or by pruning. John the Baptist told people that Jesus would baptize with the Holy Spirit and fire (Matthew 3:11). Malachi speaks of the Messiah as being like a "refiner's fire" (Malachi 3:2). Jesus refers to believers as being the branches and to Himself as being the vine (John 15:1-17). For a vine to produce fruit, it must be pruned. Those who are truly "pure," then, are those who have been declared innocent because of the work of Jesus and who are being sanctified by His refining fire and His pruning.
The Greek word for "heart" in Matthew 5:8 is kardeeah. This can be applied to the physical heart. But it also refers to the spiritual center of life. It is where thoughts, desires, sense of purpose, will, understanding, and character reside. So, to be pure in heart means to be blameless in who we actually are.
Being pure in heart involves having a singleness of heart toward God. A pure heart has no hypocrisy, no guile, no hidden motives. The pure heart is marked by transparency and an uncompromising desire to please God in all things. It is more than an external purity of behavior; it is an internal purity of soul.
The only way we can be truly pure in heart is to give our lives to Jesus and ask Him to do the cleansing work. Psalm 51:10 says, "Create in me a pure heart, O God, and renew a steadfast spirit within me." God is the one who makes our hearts pure – by the sacrifice of His Son and through His sanctifying work in our lives (see also 1 John 3:1-3).