In Psalm 50 God speaks saying in verses 9-12:
9 I will not take a bull from your house,
Nor goats out of your folds.
10 For every beast of the forest is Mine,
And the cattle on a thousand hills.
11 I know all the birds of the mountains,
And the wild beasts of the field are Mine.
12 “If I were hungry, I would not tell you;
For the world is Mine, and all its fullness.
God reminds us that everything belongs to Him, and so anything we give Him, is in fact His already. We are inclined to think that we belong to ourselves, existing independent of God, apart from Him and so we think when we give ourselves to Him we are giving Him something that is entirely ours to give. But the truth is every waking moment we experience and every breath we take, is only possible because of Him, because He sustains it and gives it too us. That’s how dependent we are on Him. We are not our own.
More specifically it is through Jesus that we and the world are sustained and upheld. Hebrews 1:3 says of Jesus:
3 who being the brightness of His glory and the express image of His person, and upholding all things by the word of His power,
Jesus upholds us, our life only exists as we know it because of the word of His power.
Paul speaking again of Jesus says in Colossians 1:17:
17 And He is before all things, and in Him all things consist (or hold together )
It is only in Jesus that we consist, are held together, meaning only because of His power do we physically, emotionally and spiritually exist.
God can truly say, as He does in Psalm 50:12 “ For the world is Mine, and all its fullness .”
Whatever we give Him, we are in fact only giving God what is already His.