If the Father and Son do not have a unified will and nature which can be distinguished by subsistent relations or origin (eternal generation), then the unity of God comes under threat. And specifically, the Arians threatened God’s unity by not affirming eternal generation and instead affirming some sort of relation in which the Father and Son agreed mutually to act together.
Jesus prayed in the Garden, “not my will be done but yours,” and elsewhere he says, “For I have come down from heaven not to do my will but to do the will of him who sent me.” Obviously, Christ has his own will, and the Father has his.
Nobody disagrees here. But the next step is crucial. Does Jesus mean that his divine will agrees with the Father’s divine will, or does Jesus mean that his human will concurs with the divine will of God?
Karl Rahner and Karl Barth
The latter was the obvious answer for all orthodox Christians for centuries. Yet Karl Rahner (1904–1984) and Karl Barth (1886–1968) introduced theological emphases that shifted the balance.
Rahner argued that what Christ does in his earthly ministry shows how God exists in Himself. Barth advocated for an eternal submission of the Son to the Father. So Rahner allowed for Christ’s earthly obedience to indicate an eternal relationship in God; Barth made the idea mainstream.
Many Christians today take it for granted that the Son naturally stands in a relationship of submission and authority with the Father. This position implies some sort of complexity within the divine will.