Things that possess actuality, existence, or essence....do these things include individual imaginations? Christians quite rightly claim God as a reality apart from our imaginations---as if He imparted His reality into us. We can never prove this fact, however, except through exemplorary behavior (which we seldom model) and even then, others without our belief can also behave quite well.
You say that Christians 'quite rightly' claim God is a reality seperate from our imaginations..
How do you test for this? Can you show this is "God" exists outside of our own brains?
Until you can show an objective way to measure 'god', outside of personal sensations, and observable via instruments rather than subjective experiances, you can not legitimently say 'quite rightly'.