The article analyses and assesses metaphysical personalism – a group of personalists including both Russian (L.M. Lopatin, A.A. Kozlov, P.E. Astaf’ev, S.A. Askol’dov, E.A. Bobrov) and German (G. Teichmüller) philosophers – in the context of modern discussions about the existence of God. The importance and potential of metaphysical personalism stems from its specific features: a conscious task of creating a personalistic metaphysics as a system, understanding of metaphysics as a rational philosophy, critical attitude to its own methods, use of rational controversial arguments in polemics, pursuance of metaphysical substantiation of the Christian theism. The article studies how metaphysical personalism sets and solves the problem of the reality of substance as the concept required to substantiate the existence of God. The concept of substance is given within the Aristotelian tradition as a real self-identical unity, the source and centre of active power. Personalism is particularly focused on proving the equal reality of both the substance and its actions. The concept of substance is substantiated based on the grounds of immediate consciousness (“inner experience”). Only the reality of spiritual, self-conscious individual substances is accepted. Other substances can be conceived by the analogy with the substance of a human subject. However, the criterion of differentiation of the substances of various ontological statuses remains unclear. The ontological argument requires to analyse the concept of being, which is also based on the structure of consciousness, including the substance of the self, its activity and the content of the activity. The immediate consciousness of a man is an archetype of any being. A man can directly access the reality of God in the consciousness of the own being. Therefore, the evidence of the existence of God results from the self-evidence of the consciousness of a substantial subject. Metaphysical personalism using the resources of the rational metaphysics has substantiated the existence of God through the structures and immediate experience of the consciousness of the subject.