You write, "Agnostics pride themselves on pointing out (supposed) flaws in Christian worldview but offer no alternative theories of existence ..."
Logic does not require that atheists or agnostics supply an alternative theory. If you said that life was created by an invisible pink dinosaur, it's not an atheist's job to propose another alternate theory. It's up to you to prove yours.
It is completely valid to put forth arguments that an invisible pink dinosaur did not create the universe. Logic does not demand an alternative theory.
To many atheists and agnostics, saying that God created the universe makes as much sense as claiming it was done by magic. Agree or not, you should be able to understand that.