In short, a Christian must be one who, as Paul says (I Cor 7, 29-31), uses this world as not abusing it, who buys and possesses as though he possessed not, who has wife and children as though he had them not and who builds as though not building. How is it possible to reconcile these seeming inconsistencies? By…accepting the fact that the Christian's attitude toward this earthly life is the attitude of the guest; that in such capacity is he to build, to buy, to have dealings and have conversation with his fellows, to join them in all temporal affairs--a guest who respects his host's wishes, the laws of the realm and of the city and the customs of the inn, but at the same time the Christian refrains from attesting his satisfaction with this life as if he intended to remain here and hoped for nothing better. Thus will the Christian pass through every temporal event in the right way--having every possession as though not having it, using and yet not cleaving to it; not so occupied with the temporal as to lose the eternal, but leaving behind--forgetting--the former while striving after the latter as the goal set before him.
Martin Luther, Baker Book House: The Sermons of Martin Luther Volume VII:272-285.
Wayne Knolhoff, Director of the Center for Stewardship at Concordia Seminary, St. Louis, Missouri.