The issue, in my opinion, boils down to this: Is forgiveness actual or potential?
Ponder the relationship between God and those who, at this moment, stand in a place of rebellion toward God. Are these people, in light of Jesus's death for them, already forgiven? Or is God currently withholding forgiveness, waiting for the person to respond and repent? In the former forgiveness is actual--the death of Jesus created a new state of affairs, a new reality, a reality where the wall of enmity between God and humanity has been eradicated. In the latter view forgiveness is potential--you're not yet forgiven. The death of Christ, in this view, merely opens up the possibility for forgiveness. But as things stand right now you are not forgiven.
This contrast--Is forgiveness actual or potential?--goes to the heart of the debates of what is called the "New Perspective" on Paul. Some of this debate swirls around how we render Paul's use of the phrase Pistis Christou.
What we all agree on is that pistis means "faith" in Greek and that christou means "Christ." So far so good. But in the Greek there is some genitive ambiguity concerning how the two nouns--faith and Christ--are to relate to each other. Martin Luther, and those who followed him, translated Pistis Christou as "faith in Christ." But a growing number of scholars (e.g., Richard Hays, N.T. Wright) have argued that the proper translation of Pistis Christou should be "faith of Christ."
Theologically, the translational differences go to the issue of the actual versus potential nature of forgiveness. In Martin Luther's rendering--faith in Jesus--forgiveness is potential. Forgiveness is contingent upon the act of faith. You need to believe and then, once you've done that, you are forgiven. By contrast, the New Perspective rendering--faith of Jesus--focuses upon the faithfulness of Jesus in creating a new reality. Because of the work of Christ on the cross the wall of hostility and accusation between God and humanity was finally and decisively broken down. Forgiveness becomes our new reality. A new world has been created. Everyone has already been forgiven in Christ. The call is to recognize this reality and live into it. To trust (have "faith in") what the faithfulness of Jesus has accomplished for us "while we were yet sinners."
All this to say that the doctrine of universal reconciliation is richly informed by the New Perspective in seeing forgiveness as a currently existing reality.
Because of the atoning death of Jesus on the cross forgiveness is actual. Because of the cross a new reality has been created between God and humanity. Faith is recognizing that reality and rejoicing in it.