How would you define the word trust?
It's not just believing something. You can believe something and at the same time not trust in it. I'm not sure I have a good technical definition for trust yet, but a word that helps me picture what it means I think, would be the word rest.
Jesus on the boat in the storm, sleeping. Waking up. Asking the disciples. "Why are you afraid? Have you no faith?"
It's not just knowing the strength of God, the promises of God. It's resting in them.
Quoting Calvin, "Here, indeed, is the chief hinge on which faith turns: that we do not regard the promises of mercy that God offers as true only outside ourselves, but not at all in us; rather that we make them ours by inwardly embracing them. Hence, at last is born that confidence which Paul elsewhere calls peace...It is an assurance that renders the conscience calm and peaceful before God's judgment."
Faith. Inwardly embracing God's promises. Trust. Rest.
It helps me so much in my Christian life that my chief responsibility is to rest in what God has promised me in His Word, to rest in the salvation He has provided through Jesus Christ. To remember that what God wants me to do is to trust in the fact that He is going to do me good on the basis of the work of Jesus Christ.
I start to see why sin is such a big deal. Greed, proof that I'm not resting, trusting in God's care for me. Lust, not trusting that God's plan for my sexual desires is for my best. Complaining, not believing God's working things out for my good. All these sins, they are not just actions, external, things I'm doing wrong; they are all indicators of problems in a relationship, a lack of resting, trusting, believing my God.