What do you think is the most important part of Christianity?

I should declare a (dis)interest. I am agnostic, probably atheist.

Christianity is still important for me and most of the Western World. It established the standards by which we try to organise our society. It defines the way that a lot of us try to live our lives.

There is such a thing as a Christian attitude, and Christian decency and neither have much to do with religion.

So if you are lucky enough to live in a country whose social standards are defined by an adherence to Christian principles, think yourself lucky. Most of the world is not so fortunate.

I think it is the part that too many Christians seem to practice the least. Love thy neighbor and do not judge.

The after service potluck dinner on Sundays.

Establishing what is the truth is the most important part of Christianity. The bible warned that there would be false teachings that would take some away from following in the footsteps of Jesus. At 2Tim 4:3,4 is says "For there will be a period of time when they will not put up with the wholesome teaching, but according to their own desires, they will surround themselves with teachers to have their ears tickled. They will turn away from listening to the truth and give attention to false stories."

We have the bible to help us to find the truth but we must use it. Our creator is looking for humble and right hearted persons to realize his purpose in creating the earth. It is God's will  for humans to inhabit the earth and enjoy life to the full on it.  Our having free will allows us to make the choice as wanting to submit to our creator's purpose or go at it on our own.

He loving tells us how the matter will end after those using free will have used it in line with his purpose. At Rev 21:3,4 it says "Look! The tent of God is with mankind, and he will reside with them, and they will be his people. And God himself will be with them. And he will wipe out every tear from their eyes, and death will be no more, neither will mourning nor outcry nor pain be anymore. The former things have passed away.

The most important part of Christianity is acting in harmony with what its holy text, the Bible, recommends. 

To a Christian it's their belief in the salvation of the sacrifice of Jesus. To anyone else there is nothing important about any part of it.