Another excerpt from a article I'm writing.
The character of the Christian God is more highly unique than it is often appreciated to be when juxtaposed to the gods of modern Judaism and Islam. The gods of the two other great monotheistic traditions mirrors perfectly both of these religions of the law's concepts of salvation. The god of post-Rabbinical Judaism stands out against the Christian God as a solitary monarch. As a solitary monarch, he is not by nature a gracious giver or perhaps he has limited this to the one time of affair of creation (possibly, also his act of sustaining creation). In that he is by nature a solitary monarch, the divine-human relationship must be based on law and not self-giving. Being solitary, he is not a giver by nature. Rather, he like any other monarchs holding on to his power, must enforce his will by the demanding submission. He does this through the teaching of Torah and the tradition of the elders found in the Mishnah and Talmud. His human subjects' role is simply to submit and thereby gain his favor by their obedience. If we substitute the word "Koran" for "Torah" the theology of Islam is essentially the same. As we shall see, in both cases, the rejection of Trinitarianism in favor of Unitarianism goes hand-in-hand with a belief in salvation by the law.
By contrast the Christian God’s own inner life is constituted by the receiving and giving of glory and love. In other words, God is by nature a gracious giver and a self-donator in his eternal being. His acts of giving in time are also rooted in his prior nature as a self-communicating giver in eternity. The Father eternally gives his entire being to the Son (Jon 1:1-5, Heb 1:1-5) and creates all things to end of glorifying him (Jon 8:54, 17:5, Phil 2:9-11). The Father is infinite and therefore he may give of himself infinitely and remain himself. His self-giving is therefore rooted in his infinity and glory. Due to their infinite glory in eternity, the Son and the Father are free to fully give the fullness of their being in the dual procession of the Spirit. In his Incarnation and earthly life the Son is therefore free to give all glory to the Father (Matt 11:25, Jon 17:1-5). In time, the Father then exalts the Son and the Spirit glorifies the Father through creation’s worship of the Son (Jon 16:4, Gal 4:6, Phil 2:9-11, Heb 13:15, Rev 4-5).
God's own self-giving in eternity therefore go hand-in-hand with his activity in creation. Contrary to the claims of Judaism and Islam, a God who was not already a giver would never create and sustain the world. Being a non-relational being, it simply would not be in his character to do as such.
This also coheres with the shape that salvation takes in Christianity. For the Christian, God who is self-giving in eternity gives himself to his creatures in time through the Incarnation. Indeed, the Lutheran claims in the doctrine of the genus maiestaticum that he holds nothing of himself back in this event. Heinrich Bornkamm notes this is built into the very structure of Luther's thought because for him “God’s nature is based on giving.” As we noted in the first section, the Incarnation is the logical out working of the nature of the unilateral promise of the gospel. The God of the Old Testament who promises Abraham his own death if he does not bless him, must donate himself completely to his people by giving himself over to their condition in order to fulfill that promise and take the death they deserve upon himself. This means that the idea of the gospel as a unilateral promise is rooted in God's own Trinitarian character. God is also, of course absolutely sovereign over his creatures and is therefore by nature a God of law as well. His character as almighty and sovereign in himself (and over against his creation!) as well as self-giving and self-communicating means that the law and gospel are rooted in God's own eternal character as God. The divine-human interaction on the basis of the law and the gospel is the precise thing that one would expect out of the Triune God of the Bible.
In light of the coherence of the Christian claim that God who is by nature giving gives freely in the form of creation and redemption, the Jewish and Islamic claims regarding God's character appear more and more incoherent. The claim of these two traditions is that a god who is not by nature relational or giving, spontaneously becomes relational and giving by making the world seem strangely inconsistent. The incoherence deepens when we are told that in redemption that god has again reverted to his non-giving nature by insists that humans first meet his demand before he redeems them. One series of divine acts appear highly inconsistent with the others. In a similar manner, they gives the distinct impression (while being emphatically denied by both groups) that God is in a sense less than ultimate. He who cannot give of himself is clearly lacking in something. That is to say, if I have a limited supply of a thing, then I must hold on to what I have. A God who is not eternally self-giving is therefore a God who is limited in that he is limited in what he can give. By contrast, the fullness of God's Trinitarian glory makes it clear he is infinite and lacking in nothing. He can give all and still have infinitely more.