"You are me, and I am you."
It can be reasoned out.
If one has a theistic view it will be reasoned out in a theistic way.
True, though it depends on the brand of theism. A true monotheist, without any mystical inclinations, would find the idea heretical. Clearly there is meant to be a division between man and God. You get closer to God by following his law, but you're still somebody else.
Early Christianity was, I think, more akin to a Vedic atman/Brahman type thing, but seeing as this type of thinking is a threat to institutions like Churches(if everybody is God then who needs the church?), it was more or less sabotoged and the teachings wittled down to monotheism again. So out of the dozens of original gospels you ended up with the four that you could tack onto the end of the Old Testament.
It seems that a lot of people today though are more interested in the mystical or contemplative type of Theisms, so the "all is one" teaching is more approachable. Thus their attraction to Buddhist teachings of this sort.
It's a very "safe" teaching. Somewhere between monotheism and non-theism. If you end up at the pearly gates and they're trying to figure out whether or not to let you in, you can say "well I sort of believed in a God sort of thing. That's ok right?" But if you want to look good in front of your scientifically minded atheists friends, you can tell them you don't really believe in that silly God, but you can prattle on about interconnectedness and how our bodies decompose and turn into dirt and the dirt becomes trees, etc., and talk about "quantum" stuff and sound really smart. Ever notice how everybody has turned into a physicist in the last decade or so?