Robin Parry is a leading universalist. He has a rather ironic, indeed, counterproductive way, of framing the issue:
As such, we need to think very carefully about how central this debate is to Christian faith. Clearly important issues are under discussion and I am not calling for the tolerance of indifference, but is this a central matter for Christianity? Is the gospel itself under threat? Is the Bible being rejected? Are the creeds in question? Is anyone actually denying God's love or justice, say. Is mission really threatened? Is the centrality of Christ being denied? Are there any core Christian non-negotiables at stake here? I suspect that, as we look at both sides carefully and seek to understand each other better we shall find that not as much is in danger as we may imagine at first.
To illustrate: I would argue that classical Calvinism is incompatible with the claim that "God is love."
Well, if you say universalism affirms the love of God while Calvinism denies the love of God, I'd say that's pretty central. So, yes, the gospel is under threat. He himself just framed the issue in antithetical terms.