"Don't be attached to the things of the world, and don't be attached to the things of the other world, because things are things. It makes no difference whether they are of this world or the other world - attachment is the problem."

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Avoid duplicity, that is, do not let your heart be divided between attachment to God and attachment to earthly things, 'You cannot serve God and mammon' (Mt. 6:24); cling to God alone, put your trust in Him alone; for the Devil, by inciting us to duplicity, seeks himself to gain possession of our heart, which is single and indivisible.

— John of Kronstadt