The proper work of God is the work of the Gospel, that is, to create mercy and forgiveness. God makes peace, righteousness, mercy, joy, love, truth, patience, kindness, and health. God is the Creator. He creates. God creates that which pleases Him and calls it good. The Gospel.
The alien work of God is condemnation and judgment upon sin. It is the crucifixion and destruction of the old Adam; the suffering and death of Christ; the satisfaction of His justice and holiness; the punishment of sin; chastisement and discipline of His children. Justice. God’s Law.
“It is as if he were saying: “Because you scoff at the Word, the Lord is forced to do a strange work, namely, to judge and to destroy.” For the proper work and nature of God is to save. But when our flesh is so evil that it cannot be saved by God’s proper work, it is necessary for it to be saved by His alien work. Because in good times we stroll and stray from the Word, our covers have to be made narrow, and we must be disciplined by various afflictions so that we may be saved by God’s alien work; the ungodly are altogether driven by God’s proper and foreign work because they V 16, p 234 do not want to get under these narrow covers but want to stretch out in their own. Meanwhile God keeps His own by means of the cross and narrow covers and thus separates them from the ungodly. This is God’s alien work, by which He condemns the ungodly, so that we may be saved. So you see that our flesh is outwardly indulgent when it is without the cross, and therefore various afflictions are necessary to control that flesh.”
Luther, M. (1999). Vol. 16: Luther’s works, vol. 16 : Lectures on Isaiah: Chapters 1-39 (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Luther’s Works (Is 28:21). Saint Louis: Concordia Publishing House.