Quoted from Karl Rahner's Wikipedia entry-
The basis for Rahner's theology is that all human beings have a latent ("unthematic") awareness of God in any experiences of limitation in knowledge or freedom as finite subjects. Because such experience is the "condition of possibility" for knowledge and freedom as such, Rahner borrows the language of Kant to describe this experience as "transcendental."
Such is the extent of Rahner's idea of the "natural knowledge of God" — what can be known by reason prior to the advent of "special" revelation. God is only approached asymptotically, in the mode of what Rahner calls "absolute mystery." While one may try to furnish proofs for God's existence, these explicit proofs ultimately refer to the inescapable orientation towards Mystery which constitute — by transcendental necessity — the very nature of the human being.