Doubt is not the opposite of faith.  Certainty is the opposite of faith.  In this life we live by faith, and necessarily, doubt.

It was the Jews in post-Resurrection times who identified Jesus' followers by saying, "Look how they love one another".

Religion is based on relationships, those with God and those with our brothers and sisters.  We can have a holy relationship with God without the mass or any of the sacraments; though those rites are helpful to our relationship, they are not essentials.  What is essential is that we love our Creator and we show that love by loving His creatures.  The institutional church can be of great assistance to us in our endeavors (and sometimes a great hindrance) but it is not absolutely necessary for sanctity.