The question of whether the largest U.S. banks remain "too big to fail" is one that "fundamentally can only be answered in a financial crisis," Treasury Secretary Jack Lew said in an interview with Charlie Rose on Wednesday night.

Lew declined to directly answer whether any American banks are too large to fail without causing a systemic crisis like the one that rocked the U.S. in 2008. But he boasted that there is a "world of difference" between the strength of the financial system heading into the crisis and today's post-Dodd-Frank Wall Street. "We have taken decisive action," said Lew.

Last summer, Lew laid down a marker for ending too-big-to-fail. "If we get to the end of this year and we cannot, with an honest, straight face, say that we have ended too-big-to-fail, we are going to have to look at other options," said Lew. Then in December, he declared victory on that score following the implementation of the "Volcker Rule" provision of Dodd-Frank, intended to prevent banks from engaging in risky bets with depositors' money.

Although Lew said the real test of reform would be a financial crisis, he added that Dodd-Frank's enhanced oversight and measures for resolving a failed bank have made the financial sector dramatically stronger. He also cited supervision of shadow banking and plans for resolving banks with operations in multiple countries as areas that need further reform.

Lew also sounded positive notes about the strength of the U.S. economy, saying that "the last couple months have been an encouraging turn in the right direction" despite weaker-than-expected jobs reports in December and January. In particular, he claimed that the recent turn away from political showdowns over the budget and debt ceiling will help lift growth in 2014.

"What’s so important about avoiding self-inflicted wounds," Lew said, is that "an economy needs to build up some momentum.”

Both this week's relatively drama-free vote to suspend the debt ceiling and December's budget agreement will help provide stability for the economy, according to Lew.

"We’re seeing government not putting a burden on the economy that makes things harder for the American people,” he explained.