What law was passed in 1798 made it illegal to say or write anything false about the government?

There is no law in the U.S. against criticizing the government. The 1st amendment guarantees it. During the civil war, President Lincoln suspended habeas corpus. Meaning you could be arrested, detained, punished without probable cause, without trial, for any reason.

The Sedition Act of 1798 made it a crime to:

". . . write, print, utter, or publish, or shall cause or procure to be written, printed, uttered, or published, or shall knowingly and willingly assist or aid in writing, printing, uttering, or publishing any false, scandalous and malicious writing or writings against the government of the United States, or either House of the Congress of the United States, or the President of the United States, with intent to defame the said government, or either House of the said Congress, or the said President, or to bring them, or either of them, into contempt or disrepute; or to excite against them, or either or any of them, the hatred of the good people of the United States, or to stir up sedition within the United States; or to excite any unlawful combinations therein, for opposing or resisting any law of the United States, or any act of the President of the United States, done in pursuance of any such law, or of the powers in him vested by the Constitution of the United States; or to resist, oppose, or defeat any such law or act; or to aid, encourage or abet any hostile designs of any foreign nation against the United States, their people or government, . . ."

The Sedition Act was shocking to many Americans, especially the Jeffersonian Democrats coming so soon after the adoption of the US Constitution with its guarantees of free speech. The statute was so overbroad that it could include virtually any type criticism of the government whether it was true or false. The Sedition Act expired and was never reenacted.