Girlguiding UK will no longer ask new members to make a pledge to love God and serve their country.
From 1 September, the words "to love my God" will be replaced in the Promise by "to be true to myself and develop my beliefs".
The words "to serve the Queen and my community" will replace "to serve the Queen and my country".
The decision follows a public consultation that gathered the views of 44,000 people, including girls and adults, members and non-members.
Girlguiding UK said the change would help to create a space where those of all faiths and none can find a home.
"We hope the new wording will help us reach out to girls and women who might not have considered guiding before, so that even more girls can benefit from everything guiding can offer," said Chief Guide Gill Slocombe.
However, former MP Ann Widdecombe told Premier Christian Radio that it was an "appalling decision" that reflected the "me society".
"Any census of this country shows that the overwhelming majority of people classify themselves as belonging to a religion," she told the station.
"What the Girl Guide group is doing is actually trying to accommodate a tiny minority – and it is a tiny minority of youngsters who would have already embraced, on rational and thought-through grounds, atheism.
"This is a minority. Adults yes, but youngsters do not reason themselves into that position that young. I think it is an appalling decision."
The new Promise reads:
I promise that I will do my best:
To be true to myself and develop my beliefs,
To serve the Queen and my community,
To help other people
To keep the (Brownie) Guide Law.