Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;

Our Founding Fathers specifically wanted to make sure that since many immigrants moved to America in order to practice their own religion, our government would not establish a state or national religion. Thus, we have the separation of Church and State. Congress at various times has attempted to pass legislation relating to religion, but most of the times the majority of reps and senators have defended & protected this Amendment. Otherwise, if legislation passes and becomes law, it usually ends up going to the Supreme Court eventually and is deemed illegal.

or abridging the freedom of speech, or of the press,

The United States is one of a few countries where the citizens can speak or write against the government and not get arrested, only because of this Amendment.

 or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.

Amendment I is probably the most cited by Americans!