The hidden God is none other than the revealed God. God is hidden for the sake of revelation. Revelation is possible only in concealment, the revealed God must as such be hidden…. The proper place for this concealment of God – which takes place for the sake of the revelation, indeed is revelation in precisely that way – is the cross of Christ…. The hidden God is none other than the crucified God. Who is a theologian of the cross? A theologian of the cross is one who speaks of the crucified and hidden God. A theologian of glory is one who does not recognize, along with the Apostle [Paul], the crucified and hidden God alone. “Crucified and hidden alone” makes thoroughly clear that the hidden God cannot be a hypostasis in or behind God, but is the one living God who is manifest as he is concealed in the cross of Christ.
It follows that the hidden God is no product of speculation. On this point the antispeculative tendency of Luther’s early theology reaches its climax. If we may put it thus, the concept of the hidden God must be construed in a strict Christological sense. Affirming omnipresence and omnipotence of God is very pointedly described as characteristic of the theology of glory. On the contrary, the hidden God is the crucified God. There on the cross, at this one place and cloaked in deepest weakness, God becomes visible. For that reason Luther cannot warn enough against the “flighty thought” of seeking God anywhere else than in the depths of God in reality places itself above God; it is obvious that it must be rejected from the perspective of the revelation idea of the theology of the cross.
Walther von Loewenich, “Luther’s Theology of the Cross”, pages 30-31