Blake Shelton Talks Gwen Stefani's Wedding Vows to Him: 'Couldn't Believe How Hard It Hit Me'

When Blake Shelton and Gwen Stefani said "I Do" in a chapel on their Oklahoma property on July 3, every detail was lovingly, carefully planned. Stefani said yes to a Vera Wang gown and completed her look with a veil hand-embroidered with their names and the names of Stefani's sons, Kingston James, 15, Zuma Nesta Rock, 13, and Apollo Bowie, 7.

Shelton, 45, paired his signature blue jeans with a tuxedo jacket, vest and bow tie. Afterward, they celebrated with an ornately detailed, multi-tiered wedding cake, dancing and then capped the evening with fireworks.

However, Shelton says, nothing was more meaningful to him than Stefani's vows. The couple knew they were writing their vows because Carson Daly, who officiated the ceremony, asked them to. Shelton stunned Stefani, 52, with a song, "We Can Reach the Stars," that he wrote for her with Nashville hit songwriter Craig Wiseman. However, Stefani's words to him took his breath away, he said.

"She took that really hard, took it to heart and talked about it all the time," Shelton says. "She was like, "Oh my God, I'm still working on it.'"

When she read them to him during their ceremony, he understood why.

"The thing that got to me was she spoke about my [late] brother and my dad as if she had known them almost," Shelton explains. "She didn't make it seem that way, but it felt that way as she was speaking about them. She said she wished they were there and what kind of men she thinks they must've been. Man, it hit me. Like, I couldn't believe how hard it hit me. I felt like at this point, I've kind of gotten calloused. I guess I'm not because that absolutely slayed me. It's hard to even remember what else she even said. I'll never forget that feeling.

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Shelton took writing his vows just as seriously. He chose to write her a song because he said it was the last thing she would expect.

"For me, writing songs is like going to the dentist sometimes," Shelton admits. So the fact that I did that, I think it was a complete shock to her."

The writing process for "We Can Reach the Stars" was an easy one. Shelton reached out to Wiseman because he knew he needed help. The singer was driving to work at The Voice and used the time to explain his ideas to Wiseman. He knew he wanted his ode to Stefani to sound like '90s country and shared his vision for the hook. He heard Wiseman fumbling around with his phone; then, the songwriter started strumming his guitar.

"He was coming up with some of the most important stuff, which was unbelievable for me to hear," Shelton says. "I was like, 'Man, wait till I get there so we can talk about it.' But at the same time, I didn't want him to stop."

Halfway through the writing process, Shelton knew the song was exactly what he wanted to sing to Stefani at their wedding.

"Everything happened exactly how it was meant to happen," Shelton says. "It just like, 'Wow, we did it. I have a song about my wife.' It was so exciting to me."

After the wedding, an intimate ceremony that only included 40 of the couple's closest friends and family members, Stefani carefully boxed up her gown to preserve Wang's delicate work. A church bench signed by their wedding guests is in their family chapel as a personal reminder of their special day. But Shelton was so proud of his song for Stefani, he had to share it – and she approved. He released "We Can Reach the Stars" to streaming services earlier this month.

Stefani commented on socials that it was "the most amazing beautiful gift I ever received."

For more from Blake Shelton, pick up the latest issue of PEOPLE, on newsstands Friday.