The Trinitarian formula (the idea that God is One but Three) is not explicitly stated in the New Testament, although inferences to this can be seen by Christians throughout both the Old and New Testaments of the Bible. It has now become a distinguishing feature, marking Christianity out from other monotheistic religions such as Islam and Judaism, which reject the idea that God can be three.
Most denominations of the Christian religion adhere to belief in the Trinity, and Councils of Christian leaders have often required acknowledgement of it for admission. Those groups who do not acknowledge the idea of Trinity (e.g. Jehovah’s Witnesses, Mormons, Unitarians) are often considered sects for this reason.