If we must be pure in heart then we must not rest in outward purity. Civility is not sufficient. A swine may be washed, yet a swine still. Civility does not wash a man, grace changes him. Civility, like a star may shine in the eyes of the world, but it differs as much from purity as the crystal from the diamond. Civility is but strewing flowers on a dead corpse. A man may be wonderfully moralized, yet but a tame devil. How many have made civility their saviour! Morality may damn as well as vice. A vessel may be sunk with gold, as well as with dung.
Observe two things:
1. The civil person, though he will not commit gross sins, yet he is not sensible of heart sins. He does not discern the ‘law in his members’ (Romans 7:23). He is not troubled for unbelief, hardness of heart, vanity of thoughts. He abhors gaol-sins, not gospel-sins.
2. The civil person has an aching tooth at religion. His heart rises against holiness. The snake is of a fine colour, but has a deadly sting. The civil man is fair to look to, but has a secret antipathy against the ways of God. He hates grace as much as vice. Zeal is as odious to him as uncleanness. So that civility is not to be rested in. The heart must be pure. God would have Aaron wash the inwards of the sacrifice (Leviticus 9:14). Civility does but wash the outside; the inwards must be washed. “Blessed are the pure in heart.’
Thomas Watson (1620-86), The Beatitudes: An Exposition of Matthew 5:1-12, 175-76.