Utilizing resources from Martin Luther and the Lutheran tradition, this study offers an understanding of the gospel as promise as key to addressing the challenge of relating the missio Dei to a generous, constructive approach toward the religious other. In its construction of a Lutheran missiology, it retrieves and reappropriates four resources from the Lutheran tradition: the gospel as promise, the law/gospel distinction, a theology of grace as promise of mercy fulfilled, and a theology of the cross utilizing the hiddenness of God. The law of God as accusing yet webbing humanity to its Creator; the gospel as the comforting promise of mercy; and the hiddenness of God as mystifying form the overarching framework within which the Lutheran missiology presented here seeks to engage the religious other by dialectically relating gospel proclamation and dialogue. Such a view of mission shaped by promise offers the paradox of God being both revealed and hidden in the cross as a distinctive contribution to an interreligious dialogue centered on the ambiguity and hiddenness of God.