"To preserve the faith" means to protect and maintain one's beliefs, values, or religious convictions. It implies holding onto core principles and not allowing them to be compromised or lost. Full definition
One thing I had to mourn last week was the killing of perhaps my greatest darling: the persona of the Good Christian I long maintained, under the theory that it could somehow help preserve my faith. (rachelheldevans.com)
but what I'm saying is that we need to «preserve the faith while reforming the church,» maintain our principles but alter how they are delivered. (educationnext.org)
By preserving faith inviolate from the brute realities of a post-Christian history, theology has isolated faith from history, whether in the orthodox manner of Barth, or via the liberal and semi-existential methods of the dialectical theologians. (religion-online.org)