The word "heart".
"You just need to ask Jesus Christ into your heart..."
"Worship God with all your heart, and He will accept it."
"He has a good heart."
You've probably all heard things like these before, and no doubt there is some truth to them in particular circumstances. But I wonder if we really understand what we're talking about here.
The Romantics would have us believe that the heart is the seat of emotions. That may even be Freudian concept. The Romantics may even be right. But if you make the claim that the heart as the Bible speaks of it is simply equal to your emotions, I would definitely disagree.
In Hebrew, and Lane can correct me if I'm wrong, the word for "heart" means something like the core of your being, closer to the will of someone. Here is Crosswalk's definition of the word used in the Great Commandment (Deut. 6:5):
1. inner man, mind, will, heart, soul, understanding
  a. inner part, midst
    1. midst (of things)
    2. heart (of man)
    3. soul, heart (of man)
    4. mind, knowledge, thinking, reflection, memory
    5. inclination, resolution, determination (of will)
    6. conscience
    7. heart (of moral character)
    8. as seat of appetites
    9. as seat of emotions and passions 1a
2. as seat of courage
So it's more than your emotions. This has implications for a number of things. First off, we should be careful when we're quoting Scripture that uses that word to remember what it means.
Second off, in worship, when it says we should worship in spirit and in truth, we must be extremely careful not to translate that sentence into the following: "If your heart is right before God, then He will accept your worship." Such a sentence usually implies, in my experience, that if your emotions are worshipful, you're ok. But you're not necessarily ok. The common dilemma proposed is the following question: would you rather have people worship on Sunday with their hearts (meaning emotions) in it, or have all their theology correct and have a rebellious heart? This is a false dilemma, I'm convinced. To worship God with your heart means to worship God with your whole being. The Hebrew in Deut. 6:5 is a heaping up, and adding on so that nothing whatsoever is lacking. Your entire being is to worship God. You don't have the option of leaving your mind, emotions, body, or anything else not already mentioned at the door! So the neophyte who wails away at the songs with all his emotions but lacks theological understanding is no better off than the dry orthodustic theologian. We are so accustomed in America, I think, to think that in worship we must have our hearts right before God and that, if that is so, we need not worry about our minds. This is simplistic. The whole being of a man must be engaged.
This is impossible with men, but it is possible with God. You see, if you make making your heart right with God a prerequisite for worship, then you're being legalistic. If you make making your mind right with God a prerequisite for worship, then you're being legalistic. Is there a prerequisite for worship? Yes, a resting on God's grace utterly. A recognition that you are a sinner in need of God's grace. A realization that the gospel is your only hope in this life. Who was it that was justified before God? The sinner who beat his breast and said, "Be merciful to me, a sinner."
In Christ.