Answer: The promise of an addition of a Bill of Rights
Details:
The Articles of Confederation, in place prior to the ratification of the Constitution of the United States of America, had granted stronger authority to the states. Patrick Henry and other Anti-Federalists were concerned about too much power winding up in the hands of the federal government and its executive branch, thus allowing a small number of national elites to control the affairs of the USA. They feared this also would diminish the rights and freedoms of individual citizens.
The Bill of Rights, laid out in the first ten amendments to the Constitution, provided some reassurance to Anti-Federalists after the fight over ratification. The compromise which led to agreement in regard to ratification of the Constitution was called the Massachusetts Compromise, because of major opposition to ratification that had existed in Massachusetts. John Hancock and Samuel Adams (both of them anti-Federalists) were the ones who helped negotiate the compromise. The anti-Federalists agreed that they would support ratification of the Constitution, with the understanding that recommendations for amendments would follow if the Constitution was ratified. The Federalists promised to support the proposed amendments, which would outline a Bill of Rights to guarantee protection of specific rights the anti-Federalists wanted specifically asserted in the Constitution.
The US Constitution was ratified in 1788. The Bill of Rights was created in 1789 and ratified in 1791.