The whole search turns inward here: the higher awareness is found by examining the self, not by looking to any figure outside. Christ in you is stated plainly as the indwelling higher consciousness seated within every person, the light that religion projects onto a distant man in history but that actually lives inside. To prove your own self is to test whether that inner presence is awake and active. The reprobate is the one in whom it stays dormant, unrecognized and unproven, the person who never turned to find what was already there. Faith here means the direct awareness that the divine stands within one's own body. The kingdom that outward religion keeps promising elsewhere is confirmed, in the clearest words, to be already inside the one who looks.
Jesus Christ in you: the higher consciousness, the anointed awareness seated within a person's own body, the light mistaken for a distant historical figure.
Your own selves: the inner ground where the divine actually dwells, the one true place to look and test for its presence.