A selection from a letter by Martin Luther to his wife, Katie. He exhorted for her to pray for the children. The letter was written on July 26, 1540.
Pray, and have [the children] pray against that horrible Satan who most violently attacks us not only in soul and body but also in property and honor. May Christ our Lord come down from heaven and also start a little fire [see Gen. 19:24; 2 Kings 1:10; Ezek. 21:31; Luke 9:54] for the devil and his companions which the devil would be unable to extinguish. Amen.
Luther's Works, Letters III, Vol. 50, edited by J. J. Pelikan, H. C. Oswald and H. T. Lehmann, Fortress Press, letter # 290.