I left Christianity (partly) because I came to the conclusion that I couldn't practice the true Christianity of the Bible and remain a moral person. If you don't practice true biblical Christianity, how can you call yourself a Christian?
I believe we have no objective purpose in life because I don't believe we were created. However, I think it's far more rewarding to gain purpose subjectively from those around you or from things you choose to do in life than gaining purpose from a deity or from a religious text.