The difficulty of the whole conception in its application to God was felt and expressed. Some philosophers, as we have already seen, denied that such an application was possible. The tide of which Neo-Platonism was the most prominent wave placed God beyond _ousia_. Origen meets Celsus’s statement of that view by a recognition of the uncertainty which flowed from the uncertain meaning of the term.[565] The Christological controversies of the fourth century were complicated to no small extent from the existence of a neutral and conservative party, who met the dogmatists on both sides with the assertion that neither _ousia_ nor _hypostasis_ was predicable of God.[566] And, in spite of the acceptance of the Nicene formula, the great Christian mystic who most fully represents Neo-Platonism within the Christian Church, ventured more than a century later on to recur to the position that God has no _ousia_, but is _hyperousios_.[567] Even those who maintained the applicability of the term to God, denied the possibility of defining it when so applied to Him. In this they followed Philo: “Those who do not know the _ousia_ of their own soul, how shall they give an accurate account of the soul of the universe?”[568] But in spite of these difficulties, the conservative feeling against the introduction of metaphysical terms into theology, and the philosophical doctrine of absolute transcendence, were overborne by the practical necessity of declaring that He _is_, and by the corollary that since He is, there must be an _ousia_ of Him. But when the conception of the one God as transcending numerical unity became dominant in the Christian Church, the term _homoousios_ (ὁμοούσιος) was not unnaturally adopted to express the relation of God the Father to God the Son. It accentuated the doctrine that the Son was not a creature (κτίσμα); and so of the term as applied to the Holy Spirit. Those who maintained that the Holy Spirit was a creature, thereby maintained that He was severed from the essence of the Father.[569] The term occurs first in the sphere of Gnosticism, and expresses part of one of the two great conceptions as to the origin of the world.[570] It was rejected in its application to the world, but accepted within the sphere of Deity as an account of the origin of His plurality. But _homoousios_, though true, was insufficient. It expressed the unity, but did not give sufficient definition to the conception of the plurality. It was capable of being used by those who held the plurality to be merely modal or phenomenal.[571] It thus led to the use of another term, of which it is necessary to trace the history.