Assuming that the so called ontological proof is conclusive (at least in some of its versions) the article tries to justify the thesis that the idea of incarnation belongs to the idea of God, what in consequence implies that His incarnation was necessary. Even though God freely created the world incarnation was necessary in order to reconcile His infinite perfection with finite and imperfect relation (this idea was already presented by some medieval theologians and philosophers). From God's point of view incarnation is an atemporal 'event', but for human beings it had to be something temporally determinate. The aim of incarnation was to give humans a moral paradigm which would show them how to overcome egoism (conatus) belonging to the essence of every personal being. In this way we can say that we not only a priori know that God as a prefect being exists but that we also know that it was necessary for Him to participate in His creation. The second part of the article aims to show that the Christian belief that Jesus was (is) God incarnate can be grounded in a reasoning similar to that presented by Rene Descartes as a justification for our 32 Stanislaw Judycki conviction that the so called external world really exists: God cannot lie - which via ontological proof follows from His perfection - and from this we may conclude that He would not allow that what is believed by Christians as His incarnation has turned out something illusory.