Mevlana Rumi’s verse of the Masnavi:

Who is the unbeliever? The one (who is) ignorant of (spiritual) master’s faith.  Who is the corpse? The one (who is) unaware of the (spiritual) master’s soul.

(ref: Masnavi Book II - 3325)

17th Century Commentary by Anqaravi:

“The unbeliever [kâfir] exists both according to the Way [tarikat] as
well as to the Law [sharî`at]. The unbeliever, according
to the (religious) Law, is someone who completely denies God and
everything which is on the side of God (such as) the prophets and
messengers and whatever appears from them. And according to the
people of the Law, any time someone denies on the inward (level)
but affirms on the outward (level), they call him a believer [=
because the Law judges who is a Muslim by what he says and
does].

However, the people of the Way, who are Truth-seeing
(and) don’t rely only on the speech and actions of someone, don’t
(automatically) recognize him as a believer. But among them an
unbeliever is someone who is unaware of the faith of the spiritual
master– who is the inheritor of the Prophet– and (who) remains
deprived of having a faith like the faith of the spiritual master.”