Why do people stray from it if it were the original Christian doctrine? How does one know one's Christian walk is true with thousands of Christian sects each purporting to be the truth and damning other sects to hell? Why do we put so much faith in our own opinions and call it God's opinion? Doesn't Christianity claim value to humility? How does thinking our opinion is God's opinion classify as such?
Roman Catholicism does NOT have the original Christian doctrine. One might be able to say that the "catholic church" had the original doctrine, because the word "catholic" meant, in essence, the true church as opposed to heretics like the Arians. Actually the phrase "Roman Catholic" is a bit strange because the "catholic church" was composed of much more than Rome. But this would require some understanding of history.
Regarding the number of Christian sects, some of them are cults that deny core Christian teachings. Roman Catholicism and Eastern Orthodoxy, as well, deny at least one core teaching which is justification by faith alone. I would not categorize either as cults, but as "world religions".
Here are the core Christian teachings from the evangelical perspective:
1) the full deity and full humanity of Jesus Christ (Jesus is God and glorified man)
2) monotheism or the belief in one God
3) the doctrine of the Trinity
4) justification by grace alone through faith alone in Jesus Christ alone
5) the inspiration and sole authority of Scripture as the rule of faith
6) substitutionary atonement of Jesus Christ on the cross (Jesus died in the place of the believer)
7) original sin
8) virgin birth of Jesus
9) bodily resurrection of Jesus
10) eternal reward of the righteous and eternal punishment of the unrighteous
There are theologies that violate core Christian teachings, without which a sound understanding of Jesus Christ and his ministry cannot be maintained. Roman Catholicism does this with regards to justification by faith alone, and with regards to Scripture alone (points 4 and 5).
According to Paul denying that justification is by faith alone is a damning heresy. I believe Rome qualifies as essentially the same heresy thing as the Judaizers the Galatian letter warned us about. It is a man-made, ritualistic system which claims that the observance of sacraments, amongst other things, contribute to the salvation of the believer.
There is an implicit claim that Christ's sacrifice was not enough, and it is maintained through various ways...for instance, the "treasury of merit" that can be released to help contribute toward the believers' salvation, upon the discretion of the Church. This "treasury of merit" contains the benefits of virtuous acts of Mary and the saints, which go above their personal obligations and therefore can be drawn upon by the sinner in order to justify them before God. This is an implicit denial of the sufficiency of Jesus' atoning sacrifice and his righteousness for those who are united with him through faith.
Do I believe all Roman Catholics are unsaved? No. I think some are saved but others are simply ignorant of true Christian doctrine. I believe God will lead them out of it. One of my friends is Roman Catholic and believes in imputed righteousness.
Regarding our "opinions" the above doctrines are clearly taught in Scripture so they are not a matter of opinion. They are a matter of biblical teaching. Rome's opinions are heretical, so I am not sure why you would bring up "opinions".
In fact, perhaps your struggle is a low regard for God's word, or perhaps you haven't studied it well enough to see that the above are clearly taught in Scripture.
Another issue with Rome is that it believes tradition trumps God's word, when it comes down to it. The Roman church interprets Scripture for the laity, and if the laity sees clear contradictions, they basically go with what the Church teaches, if they are a consistent, obedient Roman Catholic. So, Scripture is not the ultimate authority, but it is the Roman Catholic church's interpretation of Scripture.
It was the same way in a cult I belonged to. The founder was the one who interpreted Scripture, and if you challenged his interpretation, you would no longer be welcome and would be disfellowshipped. You had to evaluate Scripture through his interpretative lenses.