From gospel to rhythm and blues, here are some of President Obama's best musical moments. (The Washington Post)

When President Obama neared the end of his eulogy Friday for the late South Carolina state Sen. Clementa Pinckney (D), a victim of the shootings at a Charleston church last week, he paused. A long pause. It was a moment of genuine drama. Had he lost his place? Were his emotions getting to him?

Then, he started to sing -- the opening bars to "Amazing Grace." Soon, the entire congregation at the AME Emanuel Church joined him in song.

President Obama brought mourners to their feet during his eulogy of South Carolina state Sen. Clementa Pinckney as he sang a verse from the song "Amazing Grace." (The Washington Post)

It was a moment of considerable weight and significance: A black president leading a congregation in song at a place where nine black people were murdered by a man with the apparent goal of starting a race war.

And, it served as the coda to Obama's single best week as president -- a week filled with developments, both practical and symbolic, that will reverberate well beyond not only this week or month but his entire presidency.

The week began with Obama winning a trade fight over fast-track negotiating authority that looked to be on thin ice even a week ago. He did so by pulling off something even more remarkable and unlikely: successfully collaborating with Republican congressional leaders to find a path to passage of a rare shared priority.

While fast-track authority for Obama is not the same thing as a successfully negotiated Trans Pacific Partnership (get smart on all the trade deals here) it preserves the possibility of that 12-nation deal coming to fruition and provides Obama a bit of momentum stateside as well. If Obama is able to help make TPP happen, that will be a major foreign policy achievement with consequences lasting well beyond his presidency.

Then came the Supreme Court's ruling Thursday that upheld the subsidies for low- and middle-income Americans using the federal marketplace under the Affordable Care Act. That judgment, the second time the court had upheld a provision of Obamacare, ended perhaps the last major hope of anti-ACA forces to defund or discredit the bill.

Speaking at the White House, President Obama declared the Supreme Court ruling on health-care subsidies a win but said there is still work to be done. Here are key moments from his reaction to the ruling. (AP)

Obama did everything he could to avoid spiking the football in a statement following the court's decision. But whether he came out and said it or not, the court's ruling on Obamacare validated what is, without question, the defining policy accomplishment of Obama's time in office. Had the court decided the other way, the legacy of Obamacare would have been deeply muddled -- and it might not have even survived in anything close to a recognizable form. Given how much Obama and his party had lost in the fight for the law, that would have been disastrous. Winning, on the other hand, was a massive affirmation.

Twenty four hours later, the court was back at it -- legalizing same sex marriage nationwide. Obama was a late-arriver on the issue, without question. He supported only civil unions during his 2008 campaign and it wasn't until May 2012 -- as his race for reelection neared -- that Obama finally came out in support of gay marriage.

But even prior to Obama's own public statement in support of same-sex marriage, his administration was taking actions that led to Friday's ruling. In 2011, the Justice Department announced it would no longer defend the Defense of Marriage Act; in June 2013, the Court struck DOMA -- a decision that set things in motion for Friday's ruling.

President Obama lauded the Supreme Court ruling in favor of same-sex marriage at the White House Friday. Here are key moments from President Obama's speech. (AP)

And Obama carved out time in his second inaugural address to express his belief that allowing gays and lesbians to marry was part of the greater American movement to freedom and equality. "Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law for if we are truly created equal, then surely the love we commit to one another must be equal as well," he said that day.

On Friday, in remarks delivered after the marriage ruling, Obama returned to that theme. "Today we can say, in no uncertain terms, that we have made our union a little more perfect," Obama declared.

Then Obama got on a plane bound for Charleston where, nine days earlier, the latest in a string of mass shooting during his time in office had been committed in the basement of a famous African American church.

The speech Obama delivered, easily one of his best few as president, was a stirring appeal to the redemptive power of grace. It was about how finding grace -- even in tragedies like those visited on the church where he spoke -- was at the essence of who we all are. Obama touched on gun control, on race relations, on how what divides us is dwarfed by what unites us.

And then be broke into song. It was a genuine moment that will be remembered long after the 2016 election decides who will follow Obama into office. The most powerful person in the country, singing the words "Amazing grace, how sweet the sound...that saved a wretch like me" with the eyes of the country on him. It was deeply stirring and emotional -- not just for Democrats or African Americans but for Americans, period.

This was a week that will define not only Obama's second term and his presidency. This is a week that will leave profound implications on our society, setting off ripples that we may not fully grasp for years if not decades.

Obama ran as a change agent. For better or worse, this is the week he realized that destiny most fully during his time in office.