Unlike human governments, the God who created us can, of course, demand any and everything of us.

If he were then to demand a fixed percentage of our income, that would be completely within his right. Before the first advent, this may have been a tithe or some aggregate combination of tithes.

In the present age, I believe giving is an entirely voluntary matter, not to be done under coercion or compulsion, but rather accompanied by the promise of blessing:

"Give, and it will be given to you. Good measure, pressed down, shaken together, running over, will be put into your lap. For with the measure you use it will be measured back to you.”"
- Luke 6:38

The widow put her entire livelihood into the temple treasury.

God does, in truth, require all we have (Romans 12:1-2) as a living sacrifice.

The first and greatest commandment is this:

"You shall love the Lord your God with all your heart and with all your soul and with all your might."
- Deuteronomy 6:5

It is evident to me that this leaves Caesar zero legitimacy to demand anything.