The most important teaching - and the true heart of all religions - is that Reality is One.
This is often called non-dualism.
Non-dualism can be found explicitly or implicitly in most of the world's religions. In Hinduism's Advaita("not-two") tradition, it is taught explicitly: "Tat Tvam Asi" ("That, Thou Art").
Judaism, for instance, teaches it more implicitly. Judaism's mystical tradition - Kabbalah - reveals that the Sh'ma (Judaism's essential declaration of faith: "Hear, O Israel: The Lord our God is one Lord") is a non-dual teaching.
The Kabbalists tell us that nothing exists, other than God. In Psalm 139 (regarded as sacred scripture in Judaism, Christianity, and Islam) we find a beautiful, poetic expression of non-dual truth: "Whither shall I go from thy spirit? or whither shall I flee from thy presence? If I ascend up into heaven, thou art there: if I make my bed in hell, behold, thou art there."
Listed below are resources for learning more about Non-Dualism
What is Advaita Vedenta? : Non-dualism explained from the Advaita Vedanta perspective.
Center for Spiritual Awareness : An organization in the lineage of Paramahansa Yogananda - a great master in the Advaita tradition.