The country superstar sat down with ET's Cassie DiLaura at his album release party at the iHeartRadio Theater in Burbank, California, on Thursday, where he touched on their special and spiritual connection and new collaboration, "Nobody But You."
"Gwen was crying when we even just talked about singing it together at all," Shelton said ahead of ET's live stream of the concert. "I actually got the song through Gwen because she had been working with Shane McAnally. He told her that he had a song he needed to get to me and sent it to her first and she sent it on to me. I listened to it and I knew, 'Oh my god, I gotta cut this song. This song's incredible.'"
The song -- which is featured on Shelton's new album, Fully Loaded: God's Country -- wasn't originally created as a duet, but Shelton envisioned it as one and knew that Stefani would be a great fit. "It just seemed to fit perfectly with our story, and musically I think it walks the line of what I can do and what she can do," he continued. "Everything just kind of aligned and came together and it's one of those once-in-a-lifetime moments."
Shelton and Stefani began dating in 2015. While the two had been familiar with each other's work, they got closer while working on The Voice in 2014. The two then went through divorces from their respective spouses and things changed.
"It didn't take long," Shelton said of knowing when Stefani was the one, adding that it wasn't until the following season of The Voice that sparks flew.
"By the second time she came around, both of our lives had basically fallen completely apart," he explained. "And little did we know it happened to both of us at the same time and we bonded over that. So somewhere in that time of darkness it ended up being the greatest moment of my life. It's amazing, it truly is."
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Gwen Stefani Tears Up Over Blake Shelton and Says He 'Saved My Life' (Exclusive)
And faith has also made the couple stronger as a unit, as one can hear in "Nobody But You."
"Both of us know that the timing, and the way she and I met and came together, it just felt like home, that's not an accident," Shelton acknowledged. "I think that the way that she and I were able to save each other's lives and get ourselves through that time it was evidence to us that God had a hand in it."
Meanwhile, as fans get to hear the new track, Shelton and Stefani are also working on a music video for the duet.
"There is no way I am going to have a duet with Gwen Stefani and not [make a music video]," Shelton cheerfully said before fawning over his lady love. "Part of what's great about Gwen is she's an incredible singer. But the other part that's appealing is she is drop-dead beautiful and if I can have her in one of my videos, people actually look at my video for once in my entire career, you're damn right we're gonna do the video."
Shelton also laughingly dismissed rumors and speculation that he and Stefani had already tied the knot in secret, sharing, "I love the rumors. Those rumors are great."
"Anybody that thinks I am married to Gwen already, I love it," he joked. "My God, who wouldn't want to be married to Gwen Stefani?"
Fully Loaded: God's Country is a compilation album that features five new tracks. The LP follows the crooner's last record, 2017's Texacoma Shore.