American professional football is the most popular sport in the United States, which is why, even though it is the offseason for the National Football League, two of its stars are making headlines.

Matt Birk, who just retired as the starting center for the Super Bowl champion Baltimore Ravens, is causing controversy because of his refusal to attend the team's visit to the White House due to his pro-life beliefs.

Tim Tebow, the unemployed Christian quarterback who is arguably the most popular, yet most-debated, figure in American sports, is drawing media attention because of speculation about his future.

Birk played 10 years with the Minnesota Vikings before garnering a Super Bowl ring with the Ravens.

He told Minneapolis radio station KFAN why he didn't attend the ceremonies at the White House, hosted by President Barack Obama.

"I would say I have great respect for the office of the presidency, but about five or six weeks ago our president made a comment in a speech and he said, 'God bless Planned Parenthood.'

"Planned Parenthood performs about 330,000 abortions a year," said Birk. "I am Catholic. I am active in the pro-life movement and I just felt like I couldn't deal with that.

"I couldn't endorse that in any way."

Birk said he was confused by Obama's statement.

"For God to bless a place where they're ending 330,000 lives a year? I just chose not to attend."

Birk and his wife have been active in the pro-life movement. He spoke at a pro-life rally two years ago and called it a "life changing experience," the National Catholic Register reported.

His wife Adrianna volunteered at a pro-life pregnancy resource center across the street from a St. Paul, Minnesota abortion clinic. The center did sidewalk counseling and helped women bring their babies to term.

Tebow is still in the news because, even though NFL teams are practicing, he has yet to receive an invite to a team's training camp.

He was let go by the New York Jets over a month ago after a disappointing season in the Big Apple.

The quarterback was traded to the Jets from the Denver Broncos, a team he led to the playoffs despite a poor throwing motion and mediocre statistics.

In the midst of his playoff run, Tebow became a folk hero for his last minute victories and outspoken Christian faith.

When he scored, he would kneel in a prayer stance in a pose which became known as "Tebowing."

Vinnie Iyer of the Sporting News wrote this week, "Tebow shouldn't stop following his life path, even if that means he must settle for being more like Billy Graham than Otto Graham."

Otto Graham was a Hall-of-Fame quarterback for the Cleveland Browns who, like Tebow, could also run with the football.

Iyer notes that rumors have been rampant about Tebow's future. These rumors have included interest by other NFL teams and alternative leagues.

Some say other teams don't want Tebow because he brings with him a lot of distracting publicity.

"If there's one passion that equals Tebow's love for the game," said Iyer, "It would be his desire to do good works through his Christian faith.

The quarterback heads the Tim Tebow Foundation, whose mission is to "bring faith, hope and love to those needing a brighter day in their darkest hour of need."

Iyer said that Tebow already has plans for a children's hospital in the Philippines, where he was a "missionary kid."

Iyer said that comparisons to Billy Graham are not far-fetched. The evangelist started his career at 29, while Tebow is 25.

Kevin Hester, a Michigan pastor who even offered Tebow a job, told Iyer, "I don't know if can be the next Billy Graham, but he already has great awareness.

"Even if he has a small part of Billy's following, he can make a big difference."

Tebow topped Forbes list of most influential athletes in May.

"Tebow won't ever have as big a stage upon which to build as he does now," said Iyer.

He added, "Outside the NFL, his legacy can endure for a long, long, time, even if he doesn't play another down in any league."