Following the First Council of Constantinople, the Son is described as eternally begotten by the Father ("begotten of his Father before all worlds"). This generation does not imply a beginning for the Son or an inferior relationship with the Father. The Son is the perfect image of his Father, and is consubstantial with him. The Son returns that love, and that union between the two is the third person of the Trinity, the Holy Spirit. The Holy Spirit is consubstantial and co-equal with the Father and the Son. Thus, God contemplates and loves himself, enjoying infinite and perfect beatitude within himself. This relationship between the other two persons is called procession. Although the theology of the Trinity is accepted in most Christian churches, there are theological differences, notably between Catholic and Orthodox thought on the procession of the Holy Spirit (see filioque). Some Christian communions do not accept the Trinitarian doctrine, at least not in its traditional form. Notable groups include the Jehovah's Witnesses, Mormons, Christadelphians, Unitarians, Arians, and Adoptionists.