How do you think about the largest human identity group?
Christians say we are all children of God.
The biggest secular option is to be a cosmopolitan citizen of the world.
The latter has the advantage of seeming like an adult and modern identity. But it has the disadvantage of depending on a universal state that one could be a citizen of, which clearly does not exist.
The best modern Christian universal identity I can think of is something like co-worker with God. This works for Jews and Muslims, too.
(This is a half-formed idea).