"It has always been quietly a part of me," she told reporters at the film's Los Angeles premiere Wednesday at the Arclight Hollywood. "I'm still getting used to the idea of talking about it."
Later, while posing with bishop T.D. Jakes on the red carpet, Garner shared, "We went to his services, and I have been dreaming about them ever since!"
Wearing a strapless pink dress with black floral overlay and strappy black-heeled sandals, the 43-year-old also didn't hesitate to open up about her children with estranged husband Ben Affleck – Seraphina, 7, Violet, 10, and Sam, 4.
The actress tells PEOPLE that "if someone tells me that they see something special – if they really get something special about my kids – that is the nicest thing they can say to me."
At the screening, Garner happily greeted friends and fans, even posing for pictures.
In Miracles from Heaven, Garner plays a mother whose daughter is miraculously cured from a severe digestive disorder after falling 30 feet inside a hollow tree – the real-life story of Texas mom Christy Beam, who was also at Wednesday's premiere.
"I love the Christy Beam story, I love the story of hope, I love the story of encouragement," Garner told reporters. "I love how inspirational this story is, and I love being a part of putting her story out into the world."
Those affiliated with the film agree the star was the perfect choice for the emotional role, with director Patricia Riggen also calling the actress "a beautiful person."
"I not only looked at her talent, but what a beautiful soul that she has," Riggen said on Wednesday of casting Garner. "That was a very important aspect for this character. She was going to play a mom of three, and she was going to play such a loving character. She needed to be so loving with the new actresses that she was going to play against. So, that generosity, that big heart that she has, I thought was a very important part in order to cast her."
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Added her on-screen husband Martin Henderson, "I just have so much respect for her as a human being, first and foremost, and then as an actress."
"She is just such a kind-hearted, pure person with so much warmth," he continued. "Whenever you meet people like that in real life it's hard, because they are so big hearted it melts your heart. I think, being around her, it was just a natural affinity that we had for one another as friends, and as workmates, and I am so glad, because that is something that you can't always count on. That is a miracle," he added. "You couldn't have planned that."