I don’t see how.
Sure they can be a Christian ... just like the Bible says in Romans!
Romans 10 ...
8 But what does it say? “The word is near you, in your mouth and in your heart” (that is, the word of faith which we preach):
9 that if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved.
10 For with the heart one believes unto righteousness, and with the mouth confession is made unto salvation.
11 For the Scripture says, “Whoever believes on Him will not be put to shame.”
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Being a Christian doesn’t mean that you know it all, get everything right, or are right about even other things in the Bible (I know a lot of Christians wrong there) ... but a person is a Christian on the basis of what the Bible says ...
Sure, a liberal, even a very Progressive liberal, can be a “Christian”. All they have to do is dress up neatly every Sunday, attend services, place a generous contribution in the plate when it passes, and even get up and partake of the Communion and other rituals.
But like the Current Occupant, they never have to hear or take note of what the pastor or other congregation members may be saying. In their minds, they are engaged in what the Islamic world calls “taqiya”, to use the Shi’ite Arabic word, a form of religious dissimulation, or a legal dispensation whereby a believing individual can deny his faith or commit otherwise illegal or blasphemous acts, especially while they are in fear or at risk of significant persecution.
Now, Progressives, in fact most liberals, DO have a de facto religion, the Religion of Man, or secular humanism, which was given widespread infamy during the French Revolution, based on Voltaire’s espousal of the concept. Nominally a Catholic, openly an atheist, and yet, somewhere, wanting a supreme being into whom he could place his faith, but failing that, Voltaire evolved the doctrine of the supremacy of Reason, a sort of non-divine god-like power, but one which keeps out of the way of determined men.
And the fosters of the French Revolution were nothing if not determined men.