I know conservative/libertarians who believe in universal salvation.

The trouble with saying "wonders where they got he idea" and "one knows of one's faith" is that the history isn't clear; it's quite bizarre. The modern evangelical Christian in his or her current form is really less than a century old. The early church had several different variants from Gnostic to Pauline Christianity.

You also had the Nazoræans, the Hebrew Gospel and the Book of Adam, none of which resemble the four gospels known today as canonical. On my bookshelf I've also got the Nag Hammadi library with things in it like the Gospel of Thomas (similar to the 4, but written like Proverbs with some significant differences).

Emperor Constantine style Christianity is really what set the tone and baseline morality for what we have today, but it wasn't a straight line to where we are today.