A member of the Orthodox Church, he is noted as one of the foremost Christian apologists, arguing in his many articles and books that faith in Christianity is rational and coherent in a rigorous philosophical sense. William Hasker writes that his "tetralogy on Christian doctrine, together with his earlier trilogy on the philosophy of theism, is one of the most important apologetic projects of recent times." While Swinburne presents many arguments to advance the belief that God exists, he argues that God is a being whose existence is not logically necessary (see modal logic), but metaphysically necessary in a way he defines in his "The Christian God". Other subjects on which Swinburne writes include personal identity (in which he espouses a view based on the concept of a soul), and epistemic justification. He has written in defence of Cartesian dualism and libertarian free will.