[Federalist 78]

How is it that other Christians don't share such fundamental beliefs?

Study To Show Thyself Approved Unto God (2 Tim. 2:15)

Now these were more noble-minded than those in Thessalonica, for they received the word with great eagerness, examining the Scriptures daily to see whether these things were so.Acts 17:11

"Brethren, my heart's desire and prayer to God for Israel is that they might be saved. For I bear them record that they have a zeal of God but not according to knowledge. Romans 10:1-4

All scripture [is] given by inspiration of God, and [is] profitable for doctrine, for reproof, for correction, for instruction in righteousness: (2 Timothy 3:16 cf Psalm 19:7; John 17:17; Revelation 22:18,19)

My People are Destroyed for Lack of Knowledge – Hosea 4:6

[k2blader]

Yes, excellent passages.

Leading to another question:

If a Christian's beliefs are not founded upon the Word of God, i.e. the Bible, what are they founded upon?

A man teaching from the Word is still only a man. We are instructed to test everything.