As far as the Gospel is concerned, only those who truly believe can truly receive. There are but limited exceptions to this. One might be the new convert who has felt powerful conviction along with the Love and Grace of Christ, and so is converted even though his head, for the time being, remains full of past baggage, either of the Church variety or the atheist sort. Another could be people of limited mental capacity who cannot understand or express theological matters but have genuinely felt and received the Grace of Christ.
Beyond these examples lies a desert of unconsummated false belief; people who are under the impression that they are Saved through the Gospel, but where unrelenting contrary beliefs deny or belittle the Divinity and Sacrifice of Christ. Among these is a great range, from those who worship Mary; those who depend on the Sunday Service; those who support the restoration of Animal Sacrifice in Jerusalem; those who deny that Jesus was the fulfilment of the Old Testament prophecies or discuss Christianity as “Replacement Theology”; those who hold onto evolution thereby dismissing the true magnificence and power both of God and His Son; those who rely on their Baptism as an infant; those who follow Calvin’s blasphemous view of Predestination; those who believe Salvation can never be lost, giving freedom to sin. All of these and many more fit Paul’s description of “No Gospel at all” in Galatians. He rightly expresses astonishment at the ease with which people fall for alternative beliefs that marginalise the true Gospel of Christ. There is a most severe warning here to everyone to critically review what they are relying upon. There is but one Gospel, and it is only truly received “by those who truly believe”.