On Evidence of the evil spirit and of the Holy Spirit

When the spirit of evil is in us, then, together with oppression of heart and disturbance, we generally feel a difficulty in drawing near to God in our heart, because the evil spirit binds our soul, and will not let it raise itself to God. The evil spirit is a spirit of doubt, unbelief – of passions, oppression, grief, and disturbance; whilst the spirit of good is one of undoubting faith, of virtue, of spiritual freedom and breadth - a spirit of peace and joy. 

Know by these tokens when the Spirit of God is with you, and when the spirit of evil, and, as often as possible, raise your grateful heart to the most Holy Spirit that gives you life and light, and flee with all your power from doubt, unbelief, and the passions through which the evil serpent, the thief and destroyer of our souls, creeps in.

Saint John of Kronstadt, My Life in Christ, E. E. Goulaeff (tr), Holy Trinity Monastery,

Jordanville NY, p. 37.