The following extended quote is one of the sayings from My Life in Christ, the spiritual memoir of St John of Kronstadt (loc. 383 in the Kindle edition):
Faith in God’s existence is closely connected with faith in the existence of our own souls, as a part of the spiritual world. God’s existence is as evident to the pious mind as its own being, because every thought, good or bad, every desire, every intention, word or act of such a mind is followed by a corresponding change in the state of the heart, peace or trouble, joy or grief, and this is the result of the action upon it of the God of spirits and bodies, Who is reflected in the pious mind as the sun is reflected in a drop of water; the purer the drop is, the better, the clearer will be the reflection; the more turbid the drop, the dimmer will be the reflection; so that in the soul’s state of extreme impurity or darkness, the reflection entirely ceases and the soul is left in a state of spiritual darkness, in a state of insensibility. In this state the man having eyes, sees not, and having ears, hears not. Again, in relation to our souls, God may be likened to the out air in relation to the mercury of the thermometer – with this difference, that the expansion and rest, rise and fall of the mercury proceed from the change in the state of the atmosphere; whilst, in the first case, God remains unchangeable, everlasting and eternally good and just. Whilst the soul, changeable in its relation to God, suffers changes in itself, thus it unavoidably expands and obtains peace of heart when it draws nearer to God by faith and good works, and unavoidably contracts, becomes restless and wearied, when it withdraws itself from God by unlawful acts, want of faith, and unbelief in God’s Truth.