I have often even heard hard core atheists agree to certain Scripture passages even though they refuse the rest of God's words. For instance, many liberal polititians love to quote the Scripture about helping the materially poor but refuse the perspicuitous implications of the Bible conserning the fact that we are ALL spiritually poor.

I'll try to answer your question, IMO, with another question.

Why didn't Adam and Eve believe in God's words in the Garden? It's essential to our faith and eternal life to believe in all of God's words whether uniquely inspired in the Scripture or those words that were personally delivered to our first parents in the Garden. This is why the Reformers, without dissent, agreed to "Sola Scriptura".

Since the sin of not believing in God's words that are in union with His Spirit is truly inexplicable (blasphemy of the Spirit), I sometimes wonder even if our holy omniscient and innocent God even understands the depth of this sin.