It absolutely cannot be said with certainty because it is absolutely false. Just look at what Jesus said:
Mat 13:24 He put another parable before them, saying, “The kingdom of heaven may be compared to a man who sowed good seed in his field,
Mat 13:25 but while his men were sleeping, his enemy came and sowed weeds among the wheat and went away.
Mat 13:26 So when the plants came up and bore grain, then the weeds appeared also.
Mat 13:27 And the servants of the master of the house came and said to him, ‘Master, did you not sow good seed in your field? How then does it have weeds?’
Mat 13:28 He said to them, ‘An enemy has done this.’ So the servants said to him, ‘Then do you want us to go and gather them?’
Mat 13:29 But he said, ‘No, lest in gathering the weeds you root up the wheat along with them.
Mat 13:30 Let both grow together until the harvest, and at harvest time I will tell the reapers, “Gather the weeds first and bind them in bundles to be burned, but gather the wheat into my barn.”’” (ESV)
Every church building and every tradition in Christianity has unbelievers. There is simply no way that the visible Church can be equated with the invisible Church. Belonging to a certain tradition, such as Catholicism or Eastern Orthodox, does not make a person a believer. Having true saving faith in Christ and his work is what makes a person a believer.
There are many throughout all Christian traditions who have not done so and simply think they're a true believer because their parents were, or they were baptized as an infant, or had first communion, or go to church, etc. But none of those things makes anyone, or is evidence of anyone being, a true believer.
No. No it definitely isn't. A heresy is something that goes against Scripture and is of first order importance; it's a salvific issue. The doctrine of Mary's perpetual virginity is nowhere in the Bible and therefore 100% false. While such a false teaching in and of itself does nothing to affect one's salvation, it
could lead to all sorts of other false doctrines, which
may affect one's salvation or give one a false belief that they're saved.
Because that is simply what those things mean. It doesn't need to be in the Bible to understand and comment on. The visible Church is the physical, outward expression of those who gather together, most often done on Sunday mornings, although not always. The invisible Church is the true believers, whom only God really knows and sees. The two are absolutely not the same.