An emphatic statement — either written or spoken — is called a declaration. The Continental Congress announced their intention to form a new nation when they wrote the Declaration of Independence.
To declare is a verb, but when you add the suffix tion, you transform it to a noun. A formal public statement can be called a declaration. You would be making a formal declaration of your love, if you paid to have "I Love You, Alex" put on the big screen at the game. When you travel, you may need to make a declaration — a formal statement — about any taxable items you purchased outside the country when you return.