I really don't understand Opus Dei.
As Catholics , we are supposed to be an imitation of Christ in all we do; not just Sundays.
Did Opus Dei put out some kind of mission statement?
As for your main point, secularism is a form of thought that does exclude Our Lord. We can do secular things like read a novel or go to a baseball game but the "ism" in secularism does mean that we live our lives as if God does not exist and that is the same as atheism in action, anyway.
Secularism is probably just a non-aggressive form of atheism and that's how I would define it.