Did you know that Hitler was a very sincere man? He sincerely wanted a perfect Aryan race. He thought he would have this in the German people if the Jews, homosexuals, retarded and sick could be eliminated from their midst. Of course he was sincerely wrong. He is one of the many examples in history that should make us very cautious about believing that sincerity is all that is needed. It's important but not enough. For example, in the area of history one might say that Hitler won WWII but you wold be sincerely mistaken.
It is only in the area of religion that people talk this way. Religion deals with the invisible issues of life and death, heaven and hell and all these feel spooky, uncertain and uncomfortable. We'd rather live a sincere life believe something we think is good and might work.
Just ask anyone within different faiths if all religions are the same and you'll get an emphatic "No!". Often a person is so persuaded by the rightness of their religion that they will slaughter members of another religion.
Even a cursory look at world religions will tell you that they are not all the same because they hold diametrically opposing views of what God is like.
In Hinduism the divine is impersonal though approached through countless deities and statues.
The Muslim God, Allah, is one God but impersonal to his followers. His message comes through Mohammed.
Buddhism is a religion without God at all and even without a final existence.
Christians worship the personal God and Creator who forgives people and offers supernatural aide.
The goal of our lives also vary according to our religion. If you're a Buddhist then you are aiming for nirvana -- the complete cessation of desire and personality. Muslims look forward to a sensual paradise of wine, women and song. The goal of a Christian is to know God and live with Him forever in the company of other redeemed people.
Even those religions that believe in a personal God don't agree on if and how he shows himself to us. Islam denies that Allah ever reveals himself in person to mankind. He only reveals his will through Mohammed. Christians on the other hand declare that God not only reveals his will but also His person through Jesus Christ.
Most religions assert that by keeping their teachings a person will be saved, fulfilled or reborn. But Christianity teaches that none of us can save ourselves or make ourselves acceptable to God by good works.
The principle of karma, where a person pays off their guilt through another life, is miles apart from the Judeo-Christian view that a loving personal God longs to have a relationship with his creatures and has offered grace and forgiveness freely to all who will receive it within the lifetime that they have to live.
Still, some would say that behind all the differences there is still the essence of God and He can reconcile them all. But this means that the whole question of truth gets bypassed as all beliefs go into a melting pot and we strain off the essence. No thinking person would be satisfied with this mixture, at least anyone who views truth as important.
No, we can't just sincerely believe in any religion. Like it or not we have to admit that all religions aren't the same.