From what I have gleaned so far, I would say the short answer to the question is a resounding NO! Here is where common sense and our GOD given reasoning powers begin, based on what we know from Holy Scripture. For sake of brevity, I will classify all non-Catholic Christians as Protestants and so, in my opinion, we have five major camps of humanity, (1) Catholic, (2) Protestant, (3) non-Christian religions, which are so widely varying in their beliefs and practices, that I can speak for them only briefly, (4) and there are those who have never even heard of GOD, (5) and of course Atheists who deny the existence of GOD.
1. Do all Catholics go to Heaven?
I would say no, as there are many who call themselves catholic but really are not Catholic at all in the true sense.Some do not partake of the seven sacraments. Some do not obey the ten commandments, such as their failure to attend Mass on Sundays and holy days, for one example. "Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling." Phillipians 2:12
2. Can any Protestants be admitted into Heaven?
I would say yes, but for most, through no fault of their own, they do not have the full deposit of faith as laid out in the Bible. They have some truth but not the full truth as taught by the only Church that Jesus Christ founded. This group, probably unknowing, is in some way connected to the Catholic Church through Baptism, which is a plus for them. They could attain Heaven but it seems to me that it will be more difficult for them since they are wanting in so many of GOD given graces. Seven sources of GOD's graces are plainly listed in Holy Scripture and good Catholics strive for all seven, the sacraments. Most Protestants only have two sacraments and even they could be questionable depending upon who issues them.
Common sense tells me that the more sacraments an individual receives, the better the chance that he or she will attain eternal salvation. I would have reservations for those who persecute the only Church which Jesus Christ founded since they are really persecuting Him (Acts 9:3-5) since His Church is His body and He is its head. The hardest thing to open is a closed mind, so they should open their minds to a quest in search of the truth. Romans 2:5-8 would give me trepidation if I were guilty of its message therein.
3. Can any members of non-Christian religions attain Heaven?
I would say probably yes, but I have great reservations for those violent ones who forcibly impose their religion on others.
4. Can those who have never even heard of GOD go to Heaven?
I would say they could if they followed the laws written into all human hearts by GOD. See Romans 2:11-15.
5. Can Atheists attain eternal salvation?
Please read Matthew 10:33 and then decide for yourself.