To defend a belief or keep affirming that it's true is to contend. People used to contend that the earth was flat, but eventually, when no one dropped off the edge no matter how far they traveled, the "round" theory won.
One of the meanings of contend is from the French "to strive with," and it is a literal fighting, as in "to contend with fists." Most contemporary uses of the verb contend illustrate competitions of proof or defense, where a person will contend that something is true, or better, or wrong. It is still a striving but more of a verbal kind, where what you contend is what you hope to convince others is correct.