Either they all are Christian or none of them are. For those of us on the outside of Christendom we presume anyone who identifies as Christian is one. (Granted, they may not be very good at it, but who is?)

Once you start saying that true Christians have to adhere to a more strict set of constraints, then you open the movement to hundreds of often-conflicting standards, some of which will be more popular than yours and rule you out.

More of these standards will exclude your denomination and ministry and creeds than accept and include them.

Christianity is a movement of which the White Christian Nationalist movement that dominates the US and UK (and elsewhere) is a subset. Sadly it’s a very large subset, including the ministries in thr US that serve the greater majority of parishioners. It also includes the USCCB, but also the Vatican adheres to the anti-LGBT+, anti-women platform that aligns with the White Christian Nationalist movement that is in the process of capturing the United States federal government.

Christianity as a whole includes George W. Bush and the developers of the CIA extrajudicial detention and torture program, also the USArmy and USMC unoffical Chosen Hand of God regiments parallel to the spiritual readiness programs who were sent to engage in hot-zone operations under the color of God’s authority. It also includes Donald J. Trump who couldn’t remember a single biblical passage to save his life, and CS-gassed protestors so he could walk to a church and hold a bible for a photo shoot.

Christianity as a whole also includes Universalist denominations (of which I hear there are more than the UU, the last of the Universalist major ministries) and countless hippy denominations that are LGBT+ inclusive and see women as full persons entitled to bodily autonomy, including reproductive rights, including abortion on demand. They are Christians too, though many other Christians like to assert they are not.

Christians include everyone who identifies as such, and it is up to individuals and ministries alike, laity and cloth to work out how to regard each other.