What was the most important result of Marshall's decision in McCulloch v. Maryland?
One of the most important long-term consequences of McCulloch v. Maryland (1819) was the strengthening of the legal foundations of a federal banking system. As a staunch Federalist, Chief Justice Marshall was a firm believer in a strong central government, which entailed, among other things, a federal banking system that would facilitate America's transition to a modern industrialized state.
The ruling he handed down in McCulloch v. Maryland helped consolidate the power of the federal government in general, but specifically in relation to the banking system. The legality of the First Bank of the United States—a controversial issue for decades—was strongly affirmed here due to the Supreme Court's highly creative interpretation of the "Necessary and Proper" clause of the United States Constitution.
Even though the federal government wasn't explicitly authorized to operate a central banking system, the Supreme Court nonetheless found an implicit authorization was contained in the "Necessary and Proper" clause. This ruling had enormous implications for the power of the federal government and its relations with the states. From now on, the federal government could arrogate to itself powers that were not specifically set out in the Constitution but could nonetheless be discovered there through the kind of subtle judicial interpretation displayed by Chief Justice Marshall in McCulloch v. Maryland.