The Cardinals only title came in 1925, but wasn't actually awarded to them until 1963. The Bidwill family, who has owned the franchise since the 30s, have refused to vacate this dubious title, and it is claimed that the Cardinals have been cursed ever since. They have the longest championship drought in pro sports after the Cubs:
https://en.wikipedia.org/wiki/1925_NFL_Championship_controversy
On December 6, Pottsville defeated Chicago, 21–7, to establish the best record in the league and seemed to all but officially clinch the NFL championship.[4] Before they were awarded the championship, however, they were suspended by NFL commissioner Joseph Carr for playing a team called the University of Notre Dame All-Stars in Philadelphia (winning 9–7), on the grounds that the game violated the territorial franchise rights of the Frankford Yellow Jackets.[3]
...Cardinals owner Chris O'Brien was later offered- but refused to accept- the Championship title for his team. At the owners' meeting after the end of the season, he argued that his team did not deserve to take the title over a team which had beaten them fairly.[5] It appears that his reasons for scheduling the Milwaukee and Hammond games had been not to take the title, but rather to convince the Chicago Bears to play his team again – the Bears, with Red Grange in their roster, were a very lucrative draw.[3] The NFL said it would revisit the issue later, but never did. It was only after the Bidwill family purchased the team in 1933 that the Cardinals began claiming the championship title.[4]
It is sometimes stated that Pottsville played a fairly easy schedule prior to their suspension, often facing teams that were less than full strength from playing the day before in Frankford, making Pottsville's case less sympathetic.[3] However, the Maroons' final three games were against the Green Bay Packers, who finished the year at 8–5–1, the Yellow Jackets, who had beaten them earlier in the year and finished 13–8, and the Cardinals.[6] Furthermore, Pottsville had beaten Chicago, proving they were definitely a premier team.
By 1963, the NFL appointed a special commission to examine the case, but voted 12–2 in favor of continuing to recognize the Cardinals as champions. The lone dissenters were Art Rooney and George Halas, the then-owners of the Pittsburgh Steelers and the Chicago Bears, respectively.[3] In 2003, the issue was brought up again during the league's October owners meeting. However, the NFL voted 30–2 not to reopen the case, with the lone supporters being the Pittsburgh Steelers and the Philadelphia Eagles, the league's two Pennsylvania teams.[7] Ironically, Philadelphia's franchise is the direct successor to and is the same franchise (although, in league records, not the same team) as the Frankford Yellow Jackets, the very team that filed the protest that resulted in the ruling in the first place; the Eagles replaced the Yellow Jackets after the latter went bankrupt and ceased operations.