Oy vey! Christians are not a remnant in the US. "The most popular religion in the U.S. is Christianity, comprising the majority of the population (73.7% of adults in 2016), with the majority of American Christians belonging to a Protestant denomination or a Protestant offshoot (such as Mormonism or the Jehovah's Witnesses.)" wikipedia
There is a very important clause in the First Amendment that guarantees freedom of religion. Never forget that! If you live in the US, you are guaranteed to worship as you see fit. Believing otherwise is the source of violence towards Jews, Muslims, and others.
I have no idea what you mean by saying that we "can no longer advance a Christian agenda". I am a very serious, devoted Christian and disagree with what I believe you mean by that phrase. It seems to me to be synonymous with extreme right-wing politics, which I find to be abhorrent and anti-Christian.
Finally, the only people that say that "we now apparently have the option to either join or reject, but not to change" are those who don't vote and/or participate in democracy.