Characters in the series had what they believed were facts -- we (the viewers) were never able to independently confirm those facts, until the point where Starbuck was resurrected with a miraculous explanation.
The Lords of Kobol are treated as both historical and supernatural beings all throughout the series, and there's a number of supernatural/divine events that can be attributed to them - especially Roslin's insights under the influence of Chamala root.
In Kobol's last gleaming, Roslin correctly identifies the ruins of City of the Gods even though she never saw it before.
In the same episode, she surmises that they need to bring the Arrow of Athena from Caprica to her Tomb on Kobol to correctly find the way to Earth. In Home, this is proven true. Also in Home the Colonials come across the field from where their predecessors left Kobol. The Eight that would later become Athena gives an accurate description of the Colonials' retreat.
There's no doubt that the Lords of Kobol are real. You can possibly debate whether the Cylon God exists or not, but all the evidence points to Him existing. It's kinda funny, because I can remember people being excited about the notion that God existed in series 2, and then complaning that God existed in series 4. Not saying that you
were one of those people, but I remember some very amusing comments along those lines when the show was on the air.
Characters claiming god's existence is not the same as the writers proving his existence by showing us a miracle. Resurrecting Starbuck in a miraculous way, thereby showing us evidence of an interventionist god, changes the series from being a religious/spiritual one that explores faith and belief to being one about... well... about an interventionist god.
I get where you're coming from, but you're either misremembering or misinterpreting events from early in the show's run. They don't say for sure that God exists until the end of the show, but there were plenty of suggestions and hints before that. One of the first lines in the first regular episode is "God has a special plan for you"...