Muslims have explicitly rejected Christian doctrines of the Trinity from an early date. In Christianity, the doctrine of the Trinity states that God is a single essence in which three distinct hypostases ("persons"): the Father, the Son, the Holy Spirit, exist consubstantially and co-eternally as a perichoresis. Islam considers the concept of any "plurality" within God to be a denial of Tawhid or monotheism]]. Tawhid is Islam's central and single most important concept, upon which a Muslim's entire religious adherence rests. Shirk, the act of ascribing partners to God – whether they be sons, daughters, or other partners – is considered to be a form of unbelief in Islam and is considered the worst sin in Islam. The Quran repeatedly and firmly asserts God's absolute oneness, thus ruling out the possibility of another being sharing his sovereignty or nature. In Islam, the Holy Spirit is believed to be the angel Gabriel.