You make the Church to be a creature of US Law, when it was before it. The Catholic Church has existed here in what is now the US since 1513. It certainly doesn't operate at the mercy of US Law.
That is a very pious affirmation. Holy Mother Church only operates under the mercy of God. Since she is not underground in America as she must be in China, she functions within the corporate and legal structures of the nation and is permitted to function by the State as an untaxed entity through her several apostolates, orders, and ministries. As long as she is 'above ground' and functioning within the legal and governmental system of the United States her resources and fuctions are not sovereign.