The doubtful value of miracles as a means to instill lasting faith is something I've often noted in Scripture; consider the Hebrews who worship a Golden Calf when only days before they'd been guided by pillars of cloud and fire, or the disciples worrying about food after witnessing the feeding of the five thousand. Perhaps the Resurrection is the one exception; but even there, the living God must come and dwell in our hearts for faith to bloom.
All this came to mind once again as I pored over the new edition we're creating of Robert Falconer. After a dramatic scene in which a character who has been wrestling with Doubt has his mind set at ease by the "spiritual testimony" of another, MacDonald writes these words:
The best that a miracle can do is to give hope; of the objects of faith it can give no proof; one spiritual testimony is worth a thousand of them. For to gain the sole proof of which these truths admit, a man must grow into harmony with them. If there are no such things he cannot become conscious of a harmony that has no existence; he cannot thus deceive himself; if there are, they must yet remain doubtful until the harmony between them and his own willing nature is established. The perception of this harmony is their only and incommunicable proof. For this process time is needful; and therefore we are saved by hope.