What Is a Plumbline?

I was just over at Everyday Mommy reading an interesting post . Which got me thinking about Faith and Religions.
It is very difficult to figure out the plumbline that everyday mommy is trying to find. Yes the Bible can be considered a very good plumbline for Christians. Just as the Torah can be considered a very good plumbline for Jewish believers. And the Koran can be a very good plumbline for Islam believers. Even Hinduism and Buddhism have plumblines for their believers.
How does one know they are using the "right plumbline"? For me I would have to say you rely on Faith. Faith by definition from the Scholastic childrens dictionary is 1. Trust and confidence in someone or something. 2. Belief in God. 3. A Religion.
As A Christian I base my Faith in having trust and confidence in the belief that God exists and in his word as written in the Bible. How do we know what is written is the truth? We have to have Faith. Faith is an ever evolving process also and no two people are ever at the same stage in their journey with their Faith. It is impossible to be, given how each person is different and has experienced different events in their lives. Even if they experienced the same event, how they dealt with or reacted to it is rarely identical.
How do we know which Religion is the "right" one? If you believe what is written in the Bible, you believe Christianity is "right". If you believe what is written in the Torah, you believe Judaism is "right". I am sure you get the picture of what I am trying to say here.
If you look at the history of these religions you will realize they are all interconnected and in some ways are evolutions of one another. Islam evolved from Christianity which evolved from Judaism. Buddhism evolved from Hinduism which has been around as long or longer than Judaism. All of these religions have commonalities in their belief systems.
So I guess what I am trying to say is "the plumbline" may not be a definitive concept when dealing with religious beliefs.
My "plumbline" is to believe in Gods word as written in the Bible and try to live my life as a Christian as best I can and come judgment day I will find out whether I did the "right" thing.