Dec. 18, 2013 -- Glenn Greenwald, the former Guardian newspaper columnist who helped former National Security Agency contractor Edward Snowden reveal the United States government's massive surveillance operation to the world, said "only a small fraction" of the Snowden documents have been published and there are "definitely more to come."

"Still a lot of very significant stories that are yet to be reported," Greenwald told ABC News' Cynthia McFadden in an interview for the special, "The Year."

"We published only a small fraction of the ones that we have been given so far because we have gone through each of them and made sure that nothing we are publishing endangers human lives."

The Guardian was the first to report on the NSA's vast foreign and domestic surveillance programs, basing their reporting on tens of thousands of confidential and classified documents stolen from the NSA's internal computer system by Snowden. Greenwald's series of stories for The Guardian, which were followed by reports in other major news outlets, have since launched an international debate over privacy versus security.

"What we have done is created a debate inside the United States and around the world, one that crosses ideological and partisan lines, about the dangers of the surveillance state," he said. "I am very proud of the journalism we have done."

Beyond stark privacy concerns, Greenwald also argued that the mountain of data the U.S. government is collecting is making it harder for government officials to pinpoint terrorist communications -- the supposed point of the mass surveillance.

"The more information the U.S. government collects indiscriminately about hundreds of millions of people, the harder it is for them to find terrorists," Greenwald said.

"They're not just targeting the people who they have reason to believe are actually trying to plot harm to the United States, they're targeting everybody," he continued. "They are doing so much spying that has nothing to do with terrorists that they're actually making that job much harder for themselves."

While some laud Snowden as hero, many others have called him a traitor, and say Greenwald went too far in publishing Snowden's secret documents. Greenwald does not believe he has done anything wrong in publishing the documents and denied that he and The Guardian newspaper put lives or national security at risk, saying they have been "very judicious" in choosing which information to publish and "have consulted in every case with U.S. officials."

"Journalism is about holding people in power accountable based on the widespread recognition that those who exercise political power in the dark, in secret will not sometimes or usually but inevitably abuse that power," he said. "And the role of a journalist is to expose that which people in power are attempting to conceal? that the citizens of that country should know? so that we can have an informed and healthy democracy."

But initially, Greenwald almost missed out on what became one of the biggest stories of the year.

When he first contacted Snowden, Greenwald said Snowden insisted he install "very advanced forms of encrypted technology" so they could communicate "in confidence." At first, Greenwald was skeptical of Snowden's asking him to jump through electronic hoops without sharing much about who he was or the significance of the documents he possessed.

"I was sort of deterred by how complicated it seemed and how little I knew about it," Greenwald said. "So I kept telling him I would do it, but never really got around to it, in part because he just didn't feel comfortable telling me about what he had until I had it."

But then Snowden got in touch with Laura Poitras, a documentarian Greenwald had worked with on previous projects, and asked Poitras to help persuade Greenwald. Eventually, Greenwald agreed to talk with Snowden under his conditions.

"[Snowden] was very concerned that the government would find out what it was that he was planning on doing before he got a chance to meet with us and turn over the materials," Greenwald said. "I remember at the time thinking that he was probably a little paranoid in thinking there was this massive surveillance state that would be monitoring what it was that we were doing."

"Then once I did get the documents, and was able to read through them, and report on them, I realized that actually it was a byproduct of my ignorance, not his paranoia -- that we really do live in a kind of a surveillance state and he was quite right to be that worried."

Before meeting with Snowden, Greenwald said he asked him to send some of the top-secret government documents he claimed to possess to prove what he had was real. When he received the first batch -- about two dozen documents or so -- Greenwald remembered "barely being able to breathe" because "they were so overwhelming." He then immediately got on a plane to Hong Kong to meet Snowden, who gave him thousands more.

"Once I saw the ... full first set of the archive that he provided, the thousands and thousands of top-secret documents, that's when I knew that this was the most significant leak in national security history," Greenwald said.

Days after the first reports were posted online, Snowden revealed himself as the source of the leaks in a video. He later slipped out of Hong Kong to Russia, where he was granted temporary asylum.

Greenwald believes the Internet, where huge amounts of information can be downloaded in minutes, is what made the Snowden leak possible and the evolution of social media into a sort of information weapon helped the leak spread quickly.

"[The Internet has] really made it almost impossible for the United States government to safeguard huge amounts of information that it wants to keep secret from its own population," Greenwald said. "What the Internet and social media, in particular have done, is it has enabled people like me to reach large numbers of people."

Greenwald no longer works for the Guardian and lives in Brazil. He said there is "no question" that he is under "continuous electronic surveillance," which, if true, doesn't seem to surprise him.

"That's just the nature of the U.S. surveillance state, is they try and invade anyone's communication who they think in any way is opposing what is they're doing," he said. "And that's one of the things that make it so menacing."

Snowden, living a quiet, mysterious life in Russia, is wanted in the U.S. on espionage-related charges.

ABC News' Lee Ferran contributed to this report.