Jesus, being the good Rabbi and observant Jew that He was, knew exactly where to turn when he was asked what the greatest commandment of them all was. You can see for yourself His answer in Matthew 22:37-38 or in Mark 12:29-30. His answer was based on Deuteronomy 6:4-5, the ultimate motivation for worship, the battle cry of Israel.
Hear, O Israel: the LORD our God, the LORD is one. Love the LORD your God with all your heart and with all your soul and with all your strength.
This passage would have been familiar to both the questioners of Jesus, and to those who happened to be standing by. It was referred to as the Shema, since that was the first word in the passage (translated “Hear”), meaning to listen in the sense of obeying. This was and is the central tenet of monotheism. As usual, Jesus expanded the meaning to something beyond a statement of faith. He turned it into a call to worship.
You see, the Greek word that Jesus used, and we see translated as “strength,” is iskoos or ischus. We don't really have an exact parallel English word for that, but if we did, it would be “everythingness.” What Jesus was saying is that our worship of God is to be much more than just mental ascent. More than simply proclaiming a set of beliefs. More than being satisfied that we've aligned ourselves with theological orthodoxy. These are important things, and vital to us if we're to worship “in spirit and in truth.” But here Jesus is calling for more. For all, actually.
Jesus' words demand a response; a response to God for all He is and for all He's done; a response of everything that's in us. After all, that's what worship is... a response. We didn't make it up and we can't really initiate it. We can only respond.
And there are plenty of reasons to do that.
