The 66-year-old, who has been described as the most famous openly transgender woman in the world, was speaking in the second season of her reality TV series I Am Cait.
"One of the things I really had to deal with is, I've always wondered, because I felt like God made me this way, why did he do this?" Jenner said.
"I kind of had somewhat of a revelation and that turning point was saying to myself maybe this is the reason God put me on this earth - to, number one, be open with myself about truly who I am as a person.
"There is nothing better in life to not have any secrets and living your life authentically. Nothing better than that.
"And two, maybe you can make a difference now."
She said learning about the trans community is "challenging" because her first meeting with a trans person was only nine months ago.
"The difficulty for me has been learning about the community," she said.
"I'd never even met another trans person until nine months ago, really, when I had all the girls over for dinner and we sat down and started talking about it."
The girls Jenner referred to include Geena Rocero, Candis Cayne and Jenny Boylan, who are also transgender and have featured in I Am Cait.
With the trans community facing stigmatisation by conservatives in America, many were surprised when Jenner revealed she was a Republican in 2015.
She admitted to finding it "difficult" to get a handle on US presidential hopeful Donald Trump.
"I have not figured out what Donald Trump is and so many of the issues where he's at. I think that's very difficult," the Keeping Up With The Kardashians star said.