The problem here is positioning. What exactly are you trying to position Christianity against? Judaism? Atheism? Christianity can be looked at as “Judaism with Jesus.” In that, the story that the Hebrew Bible tells has it’s most natural extension in the New Testament, which adds new ideas and new stories to continue the story of God and humanity and give a prophetic ending.
If you want to call Christianity a ‘mere’ opinion, then are you agreeing with the Jews in that Jesus wasn’t the messiah that God promised them? This would essentially make you a Jew by belief.
Or do you think Jesus and the story of the New Testament wasn’t enough, then you’re moving in the direction of Islam.
Perhaps the idea of basing a religion on the collection of writings that we today call Abrahamic scripture is dumb. Eastern religion grew out of Hinduism and it’s most modern offshoot is Buddhism. Pick your poison.
But if you’re essentially wanting to question the whole idea of religion, then you need to look much farther afield to the realm of philosophy. Epistemology concerns how we know things. Does having an experience of something make it true? So if you have an experience of divine revelation, does that make it true?
Or are you trying to pick and choose which experiences that people have can be true or not? What experiences can people have that you would consider fact and not opinion? This resolves to basic philosophical stances on monism vs. pluralism. Most materialist stances can be categorized as Material Monism, whereas most religious stances are either pluralist or subjective idealist in nature.
Personally, the concept of supervenience is the most interesting one in this space for me, and is to me the philosophical fulcrum between skepticism and belief, at least, until some smart fellow comes up with a better one.