Is it permissible to believe those who claim to know the unseen, such as Nostradamus and others, in light of God's warning against the treachery of Satan, and how do we remain steadfast in faith?

None knows the unseen save Allah, and whoever claims otherwise has disbelieved. Most of what soothsayers and diviners report about the unseen are mere conjectures and assumptions, or matters that anyone with experience of reality would expect. They may also report personal matters unknown to most people through human and jinn devils who exchange information. Rarely, a soothsayer might speak truth in what he receives from the eavesdropper before it is struck by a meteor. Some may also be correct in interpreting certain visions and prophecies found in the books of the People of the Book that have not been corrupted. As for Allah's revealing some of the unseen to whomever He wills of His servants, it is a reality that occurred in the Ummah of Muhammad and those before it.