Everyone practises moral relativism. Look at what you wrote earlier -
Christians believe some different things, but I think we are all agreed on the essentials. We are all followers of the risen Christ, we all believe in the triune God, we all believe in the forgiveness of sins, we look forward to the coming of God's kingdom of love and justice.
Let us assume that you're right and that those are the main bits that all Christians agree on and that there is a lot of disagreement over the rest. Now, those things are lovely things to believe in but none of them are anything to do with morals. They're simply beliefs. They're not even enough to build a moral structure upon as they say nothing about ones relationship with other humans - they're all about your relationship with god.
Instead, you have to look at some other source for inspiration for your moral structure. I could be wrong but I understood that that source for Christians is, utimately, the bible. And so a given Christian will pick and choose which bits of the bible they think are important, which bits are allegorical/metaphorical and so need re-interpretation, which bits only applied in the society at that time and which bits are the result of human embellishment or interpretation and so can safely be ignored. And then there is all the centuries of later apologetics that further re-interpreted the stories of the bible until their claimed message can be used to support current social situations. And you may well read other, more modern books about Jesus and god and ones place in creation and those will further influence your personal morality.
Once you start picking and choosing which of the rules you think you should follow and which ones you shouldn't then that is moral relativism.
Anyway we do all, ultimately, do what we want. If we choose a particular course of action because we think it's what god expects of us then we are making that choice because we want to follow god's rules (or, depending on our belief structure, we want to avoid being sent to hell). If we choose a particular action because it fits in with our own personal morality and we believe it is the right thing to do, we do it because we know we bear responsibility for our choices and we want to be able to look ourselves in the mirror.