‘Only the Holy Spirit can purify the intellect, for unless a greater power comes and overthrows the despoiler, what he has taken captive will never be set free (cf. Luke 11:21-22).
In every way, therefore, and especially through peace of soul, we must make ourselves a dwelling-place for the Holy Spirit. Then we shall have the lamp of spiritual knowledge burning always within us; and when it is shining constantly in the inner shrine of the soul, not only will the intellect perceive all the dark and bitter attacks of the demons, but these attacks will be greatly weakened when exposed for what they are by that glorious and holy light.
That is why the Apostle says: ‘Do not quench the Spirit’ (1 Thess. 5:19), meaning: “Do not grieve the goodness of the Holy Spirit by wicked actions or wicked thoughts, lest you be deprived of this protecting light.”
The Spirit, since He is eternal and life-creating, cannot be quenched; but if He is grieved – that is if He withdraws – He leaves the intellect without the light of spiritual knowledge, dark and full of gloom.’