The Sign of Christianity:

If you should see a person puffed up by arrogance and pride because he is a participator of grace, and even if he should perform signs and should raise up the dead, if he, nevertheless, does not hold his soul as abject and humble and he does not consider himself poor in spirit and an object of abhorrence, he is being duped by evil and is ignorant. Granted he should perform signs, he is not to be trusted. For the sign of Christianity is this, that one be pleasing to God so as to seek to hide oneself from the eyes of men. And even if a person should possess the
complete treasures of the King, he should hide them and say continually, "The treasure is not mine, but another has given it to me as a charge. For I am a beggar and when it pleases him, he can claim it from me." If anyone should say, "I am rich. I have enough. I possess goods. There is nothing more I need," such a person is not a Christian, but a vessel of deceit and of the devil. For the enjoyment of God is insatiable and the more anyone tastes and eats, the more he hungers. Persons of this kind have an ardor and love toward God that nothing can restrain. And the more they apply themselves to the art of growing in perfection, the more they reckon themselves as poor, as those in great need and possessing nothing. This is why they say, "I am not worthy that the sun shines its rays upon me." This is the sign of Christianity, namely, this very humility.

St. Macarius, Spiritual Homily, 15.37