What does the apostle Paul mean when he says, “Do not quench the Spirit” (1 Thess. 5:19)?

God who grants us His Holy Spirit as a free gift that constantly works within us warns us through the mouth of the apostle Paul not to quench the Spirit; that is, not to stop His work of enlightening us by resisting His work in us. However much we might sin, truly, the Spirit never departs from us. But He grieves for us, and His work in us diminishes when we cease to cooperate with Him.

St. John Chrysostom likens the gift of the Holy Spirit to the flame of a lamp or a lantern shining inside a house; if a person opens two doors facing each other, a strong draft will enter and extinguish the flame. So, he says, “For instance, has any one insulted and reviled you? You should shut the mouth; for if you open it, you add force to the wind. Do you not see in houses, when two doors stand directly opposite, and there is a strong wind, if you shut one, and there is no opposite draught, the wind has no power, but the greater part of its force is abated? So also now, there are two doors, your mouth, and his mouth who insults and affronts you; if you shut your mouth, and do not allow a draught on the other side, you have quenched the whole blast; but if you open it, it will not be restrained. Let us not therefore quench [the flame].”⁸⁷⁵

And the oil in this lamp is the working of love, for the fiery Holy Spirit continues to work, aflame within us, so long as our we cooperate with Him in our inner depths, through our love for God and people. But if, in our inner depths, we are closed towards God and people, we lose the oil of love that burns within us. And St. John Chrysostom says that when thieves enter to ransack a house, they extinguish the lamp inside it, so that in the darkness they can achieve their goal. Thus, the main strategy of Satan in assaulting a believer’s heart is to destroy the work of the Spirit within him and to deprive him of that work his whole life. St. Basil says, “he does not remain with those [sinners] who by the instability of their will, easily drive away the grace that they have received.”⁸⁷⁶