From Overcoming Fear, Worry, and Anxiety by Elyse Fitzpatrick:
Jesus really got down to brass tacks when He said, “If God so clothes the grass of the field, which is alive today and tomorrow is thrown into the furnace, will He not much more clothe you? You of little faith!” (Matthew 6:30, emphasis added). Little faith! Think about those words. The Lord equates our worry with a lack of faith.
Why does the Lord say that worry is unbelief? How does my worry reflect the level of my faith? My worryometer is also a faithometer; and in this case it isn’t my faith that’s red-lining–it’s my unbelief. Why is worry unbelief? Because it has its roots in doubt about God’s character. It questions His Fatherly care and provision. When I worry about what’s going to happen to my life, what I’m really saying is, “God, You can’t handle this. You’re either too weak, uninterested, unloving, or not smart enough to take care of my life. I’ve got to devote all my attention to sorting this situation out on my own.”
Got has directed His children not to worry; He’s classified worry as a sin. Why? Because worry flows out of a distorted or incomplete view of His nature and character. God has revealed Himself both in His creation and in His Word. We are obligated by this self-revelation to know Him as He is. Although we’ll never completely understand Him or be able to fully comprehend His nature, He’s given us everything we need to know about what we need to know. When we spend our days worrying, we’re disregarding what He’s told us about His perfect holiness, power, wisdom, and love. We’re saying, “I have to handle this because You can’t be trusted.”
Worrying is also sinful because it elevates our thoughts and abilities to a godlike position. When we worry we’re putting our trust in our thoughts and in our ability to “work things out” in our mind.