A belief in one common truth is also another important tenet. The living truth is seen as more far-reaching than national, cultural, or religious boundaries.
In Christianity, Universalism refers to the belief that all humans can be saved through Jesus Christ and eventually come to harmony in God's kingdom. A related doctrine, apokatastasis, is the belief that all mortal beings will be reconciled to God, including Satan and his fallen angels. Universalism was a fairly commonly held view among theologians in early Christianity: In the first five or six centuries of Christianity there were six known theological schools, of which four (Alexandria, Antioch, Cesarea, and Edessa or Nisibis) were Universalist, one (Ephesus) accepted conditional immortality, and one (Carthage or Rome) taught the endless punishment of the lost.. The two major theologians opposing it were Tertullian and Augustine. In later centuries, Universalism has become very much a minority position in the major branches of Christianity, though it has a long history of prominent adherents.
"After long study and experience, I have come to the conclusion that  all religions are true;  all religions have some error in them;  all religions are almost as dear to me as my own Hinduism, in as much as all human beings should be as dear to one as one's own close relatives. My own veneration for other faiths is the same as that for my own faith; therefore no thought of conversion is possible." (M. K. Gandhi, All Men Are Brothers: Life and Thoughts of Mahatma Gandhi as told in his own words, Paris, UNESCO 1958, p 60.)In Ananda Marga, a branch of Hinduism, Universalism refers to the idea that energy and matter are evolved from cosmic consciousness. Thus, all created beings are of one universal family. This is an expansion of humanism to include everything as family, based on the fundamental truth that the universe is a thought projection from the Supreme.
Although that system was initially for what Islam defines as the "people of the book" (i.e. Jews and Christians), some Muslims extend this to include Mandeans, Zoroastrians, and Hindus. Other Muslims disagree, and hold that adherents of these faiths cannot be dhimmi.
The Muslim ideal of universal brotherhood is the Hajj (pilgrimage to Mecca) prescribed by Islam. Each year close to three million people from every corner of the globe assemble in Mecca to perform Hajj and worship God. No individual can be identified as a king or pauper because every man is dressed in ihram clothing.
Unity, Religious Science, Divine Science are denominations within the New thought movement. Each teach there is a golden thread of Truth which is at the center of all religions. New Thought is an ever evolving belief system which will incorporates Truth where ever it is found, hence the name New Thought. All is God, But God transcends all.
I believe in the fundamental Truth of all great religions of the world. I believe they are all God given and I believe they were necessary for the people to whom these religions were revealed. And I believe that if only we could all of us read the scriptures of the different faiths from the standpoint of the followers of these faiths, we should find that they were at the bottom all one and were all helpful to one another. - M K Gandhi