Christian disability charities John Grooms and the Shaftesbury Society agreed to merge in November last year. A core group of trustees and all the directors of the new organisation, which becomes operational later this year, will have what employment law calls a genuine occupational requirement to accept only people of a Christian faith.
A spokeswoman for Shaftesbury said: "The details of posts to which the requirement will apply are not yet finalised."
Currently, several posts at Shaftesbury are reserved for Christians.
The only post to which this applies at John Grooms is that of the chief executive's role.
However, the spokeswoman denied that the merger would put non-practising Christians at a disadvantage. "There will be very few overlaps," she said.