God's path to union with his friends is to become a man earlier than planned.
Realignment Is Not Enough
God's efforts in realigning humans toward our proper goal of union with himself proved inadequate to enable us to reach that goal, as the wrongdoing of the unrealigned population continued rampant.
For details see Realignment is not enough.
The First Advent
The planned anthroposis of God the Mediator would occur "early", with mankind unripe. The Mediator, himself unspoiled by wrongdoing, would begin to live the fulfilled life, the first human to do so, and so give selves a foretaste of elevation. In this elevation, wickedness would be swamped and humanity would ripen.
For details see The First Advent.
The Second Advent
When man would be ripe, the Mediator would finish undoing all the ill effects of man's defection. Selves would then enter fulfillment, and the age of fulfillment begin.
Fulfillment begins with experiencing something like the Mediator's resurrection. For the deceased this is literally a resurrection from death, while for survivors it is a metaphorical "resurrection" from the spoiled life.
Why believe it?
All this rings true with me because it feels like the final piece of a puzzle. Given all the beliefs listed earlier in this creed, this one makes sense.
Objections to any Belief in a Saviour of Mankind
It is implausible that the cure of humanity's ills should depend on one man.
This is the "folly to the nations" that Paul of Tarsus mentions (1st Corinthians 1.23), known as the scandal of particularity. But it only seems foolish to those who have already rejected my view of human nature, both as originally created and as it has become; to those who see their need for union with God, and especially to those who see the plight of humanity as incorrigible wrongdoers, it all, as I mentioned above, makes perfect sense.
(Satisfaction Theory) You have overlooked an important point. God's justice was incompatible with simply overlooking his friends' previous wickedness. Justice required satisfaction, and only God himself, by becoming a man and dying, could adequately provide that satisfaction.
Anthroposis amidst wickedness entails suffering, and in this sense the Mediator pays the price for our sins. Satisfaction theory takes this metaphor too literally.
For details see The Satisfaction Theory of Atonement.
(The "Happy Fault" Theory) God's anthroposis does not just restore the original plan. It enables a higher life that is only possible because the Defection occured.
This theory arises from the mistaken idea that God's omnipotence means that everything that occurs is part of God's plan.
For details see The "Happy Fault" Theory.
(Premillennialism and some forms of Amillennialism) Your talk of ripening of humanity is over-optimistic, as if the first advent would enable us to begin building the Kingdom of God in this age. Instead, the New Testament tells us that evil will continue to dominate world affairs until Advent 2. God's friends can only maintain their testimony, invite others to join them in hope, and meanwhile do what little they can to combat evil.
As far as I know such a view would only arise from rejection (or at best a shaky grasp) of our cultural task, and to that extent is really an expression of objections addressed under that item. But I will add a few words.
It is true that until Advent 2 there will be distorted developments. But it is not true that the NT is pessimistic about what can be achieved in this age. The NT pictures the ekklesia following Advent 1 not as saved souls huddling together but as a city on a hill, a separate nation.
God's friends cannot tell, and need not know, how much or little they will achieve in this age. They only need to know that salvation has begun and that their task is to participate in it.