"There is nothing that enters a man from outside which can defile him; but the things which come out of him, those are the things that defile a man."
These words lay out a basic criterion between the true religion and all false religion. False religion is always concerned about externals, first and foremost, and exclusively. The true religion is concerned about the wicked nature of the heart, and how this issue must be resolved before a Holy God.
False religion is very easy to practice: do a few things, and one can pretend he is in God's good standing. True religion requires sorrow for heart sins, repentance, and faith in the only Redeemer of God's elect.
[Puritan quote of the day: "It is a true rule in divinity that the desire for any grace is the grace itself; for to desire to believe is faith, and true desire to repent is repentance, and true desire to mourn for sin is mourning for sin." --Christopher Love, in, "The Dejected Soul's Cure"]