Section 1. All Debts contracted and Engagements entered into, before the Adoption of this Constitution, shall be as valid against the United States under this Constitution, as under the Confederation.
The new US government established by the Constitution promised to take on all the debts racked up by the older, weaker national government of the Articles of Confederation. This signaled that the passage of the Constitution wasn't going to be used an excuse for the national government to shirk its debts.
Section 2. This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.
The Constitution is the "supreme law of the land"; no other law passed by any of the states can trump the Constitution. Interestingly, treaties ratified by the Senate also gain the status of "supreme law of the land." So if the terms of a treaty ever contradict state laws, the terms of the treaty take precedence.
Section 3. The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.
All government officials, elected and appointed, must swear an oath to support the Constitution of the United States. However, that oath need not be religious in nature, and the government cannot require its officials to pass any test of religious affiliation to take office. The ban on religious tests was included to ensure that the US government would remain secular in nature.