Cast your burden on the Lord, and he will sustain you.Psalms 55:22
Care, even when addressed to legitimate matters, if it is carried to excess, has in it the nature of sin. Again and again Jesus exhorted His followers to avoid anxious care. The apostles reiterated the call; and it is one that cannot be neglected without involving transgression: For the very essence of anxious care is imagining that we are wiser than God and putting ourselves in His place as if we could do for Him what He has undertaken to do for us. We attempt to think of things that we imagine Him forgetting; we work to take upon ourselves a heavy burden, as if He were unable or unwilling to take it for us.
Now this disobedience to His plain precept, this unbelief in His Word, this presumption that intrudes upon His province, is all sinful. But more than this, anxious care often leads to acts of sin. If we cannot calmly leave our affairs in God's hand but attempt to carry our own burden, we will be tempted to use wrong means to help ourselves. This sin leads to a forsaking of God as our counselor and resorting instead to human wisdom. This is going to the broken well instead of to the fountain, a sin of which Israel was guilty in the past.
Anxiety makes us doubt God's loving-kindness, and so our love to Him grows cold; we feel mistrust, and in this we grieve the Spirit of God, so that our prayers are hindered, our consistent example spoiled, and our life one of self-seeking. Such lack of confidence in God leads us to wander far from Him; but if through simple faith in His promise we cast each burden as it comes upon Him and are "not . . . anxious about anything"1 because He undertakes to care for us, it will keep us close to Him and strengthen us against temptation. "You keep him in perfect peace whose mind is stayed on you, because he trusts in you."2
1) Philippians 4:6
2) Isaiah 26:3