Because defining complex things is fun.
Christianity is absolutely, completely, fundamentally about Christ. More than churches, Christians, religion, creeds, sacraments, or anything else, it is Christ.
In four ways:
Faith in Christ
Believing Jesus who he said he is - God. Trusting his goodness, and believing his revelation. This is the trigger for the other three.
Relating with Christ
Pursuing a relationship with Him. This is the most insane thing. We can have a relationship with God, by his grace and his love we can know him personally. This should revolutionise your life. This is the reward for the other three.
Following after Christ
Learning to be obedient to what he tells us. This is where all the meat of what is usually seen as Christianity hangs out - rituals, morality, churches, evangelism, etc. It’s the response to the other three.
Salvation by Christ.
We don’t save ourselves. It’s not even by our effort or righteousness that we can believe, follow, or relate to God. It’s by his grace. It’s by his grace and his sacrifice we can approach him at all. It’s the way we are saved. This is the enabler for the other three.
Everything else is secondary. And much is flexible. These four things though are fundamental, they are the core of Christianity. I’m pretty broad when it comes to denominations, even to groups and beliefs others may be wary of, but I see these as essential, these are what makes christianity christianity, even when we disagree on sometimes fairly substantial doctrines.
I stole most of this commentary on Phillipians I’ve been reading, and I wrote this in 10 minutes so it needs a lot of work.