The United States Constitution expressly forbids the government to pass <em>ex post facto</em> laws, which are laws that can change the legal consequences of actions in the past. In some countries <em>ex post facto</em> laws are acceptable in some situations, but in the U.S. this is such an unacceptable thing that it seemed obvious and unnecessary to include in the Constitution.
One of the major obstacles of Kennedy's campaign for the presidency was his "religion," since he was running as a Catholic, and Catholics faced a great deal of opposition in the US during this time.
