What is God's criterion for reckoning with someone born Christian who did not convert to Islam, and how does that align with God's grace upon someone born Muslim?

Allah does not punish a person for being born Christian. If a child of disbelieving parents dies before reaching puberty, that child will be among the people of Paradise, based on the Almighty's saying: ﴿And We would never punish until We had sent a messenger.﴾ and His saying: ﴿And no bearer of burdens will bear the burden of another.﴾ The Prophet, peace and blessings be upon him, saw in his vision children around Abraham, peace be upon him, and they were the children of mankind, meaning the children of all human beings. So the correct view is that the children of polytheists are among the people of Paradise. A person is held accountable for persisting in disbelief after reaching puberty, after the evidence has been established against them, and after they have rejected it.