While I agree that many of us are headed from different parts of the city or countryside, if we are Christians or seekers we are all headed to the same destination. So while our satnav path may look different, there are inevitably similar experiences along the way from which we can learn. Beyond that if you are a Christian you believe the Word will apply to all of those situations. Those who study the word can therefore offer insight. I believe this also includes truths like the trinity. So in those sense I would say no it's no purely internal. That being said yes, faith is head knowledge acting in anticipation from the heart as a relationship between you and God.

"It's a fundamental rule found everywhere - there is a softer voice within that will speak to of it but you have to ask it to tell you and then have ears to hear it." But without experience and until you learn to discern the softer voice you must test it against scripture, to know whose it is.