Genesis 3:5 God knows that when you eat from it your eyes will be opened, and you will be like God, knowing good and evil.
Satan wanted Adam and Eve to doubt the goodness of God, so he focused on what God withheld. They were free to eat from every tree in the garden, except the tree of the knowledge of good and evil (Genesis 2:17). But if God was holding something back, maybe he was not so good after all. First the devil tempted them to doubt God’s word, then he tempted them to doubt God’s goodness.
This is still effective whenever we don’t get something that we want (like someone to marry) or whenever we get something that we don’t want (like the death of a child). Whenever life disappoints us, we tend to question the goodness of God.
A missionary served God for years when his son was attacked by a swarm of bees. The attack was so fierce that his son jumped off a cliff to his death. Wouldn’t a good God have kept that from happening? Adam and Eve were the first, but certainly not the last, to question the goodness of God.
Thankfully, God proved his goodness once and for all, through the death and resurrection of his Son. Before the death and resurrection of Jesus Christ, it was less clear what kind of God we were dealing with. But when God so loved the world that he gave his one and only Son (John 3:16), his goodness was clearly displayed for all to see. Only a perfectly good God would give his Son to die for a world that hated him.
As soon as we believe that Jesus died for our sins, we give up the right to question the goodness of God ever again. If the sacrificial death of Jesus Christ does not convince us that God is good, nothing ever will. Every believer must settle this matter once and for all: God is good all the time. The devil hates this fact and wants us to doubt it, but Jesus proved this fact and wants us to trust it.