God: "I looked for someone to take a stand for me, and stand in the gap" (Ezekiel 22:30)
It does not seem proper to speak of one attribute of God as being more central and fundamental than another; but if this were permissible, the Scriptural emphasis on the holiness of God would seem to justify its selection.
Union with Christ is really the central truth of the whole doctrine of salvation not only in its application but also in its once-for-all accomplishment in the finished work of Christ.