Good morning! The casting comes from the humbling. In the verse preceding this one, Peter writes, “Humble yourselves, therefore, under the mighty hand of God so that at the proper time he may exalt you, casting all your anxieties on him, because he cares for you” (1 Peter 5:6-7). This is all one sentence. You cannot understand 5:7 without 5:6.
And, if you want to walk it back a little further, Peter reminds his younger listeners that God opposes the proud and gives grace to the humble (5:5). The clothing of humility paves the way for us to cast our cares on Him, for if we walk in our pride, do we not risk trying to handle all of our issues ourselves from our own shallow wells?
Humble yourselves! Surrender! Submit! Without this, we cannot cast our cares and anxieties upon Him! If we think we have it all together, then those cares and anxieties will stay along for the ride.
What is God doing to humble you and bring you to the feet of His dear Son? Whatever it is, lean into it! You’ll be glad you did!