The dominant form of football at the time was played at the collegiate level , but the upstart NFL received a boost to its legitimacy in 1925 when an NFL team, the Pottsville Maroons , defeated a team of Notre Dame all-stars in an exhibition game .  A greater emphasis on the passing game helped professional football to further distinguish itself from the college game during the late 1930s.  Football in general became increasingly popular following the 1958 NFL Championship game , a match between the Baltimore Colts and the New York Giants that is still referred to as the "Greatest Game Ever Played". The game, a 23–17 overtime victory by the Colts, was seen by millions of television viewers and had a major impact on the popularity of the sport. This, along with the innovations introduced by the new American Football League (AFL) in the early 1960s, helped football to become the most popular sport in the United States by the mid-1960s.