The liberty of sons is in the inward spiritual freedom of their hearts, naturally and kindly going out in all the ways and worship of God. When they find themselves straitened and shut up in them, they wrestle with God for enlargement, and are never contented with the doing of a duty, unless it be done as in Christ, with free, genuine, and enlarged hearts. The liberty that servants have is from duty; the liberty given to sons is in duty.