What makes Gerald McDermott’s book God’s Rivals distinctive is that it explores how both the Bible and early Christians dealt with other religions. In chp 6 he explores how Irenaeus’ debates with Gnosticism shed light on why God has permitted different religions.

What is the problem that needs to be faced? This is what I think it is: Can we believe a God of mercy and love would not leave a witness to his saving graces to all?
Irenaeus followed Pothinus as bishop of Lyons (in France now) and emerged into leadership following a rash of brutal martyrdoms and persecutions. Irenaeus’ theology is sketched — and that theology is difficult to sketch. Irenaeus wrote the first theology of history, following hard upon heels of what we now call the deuteronomic historian, the prophetic sense of history, and the early Christian sensitivity to “signs of the times.”
For Irenaeus, God has acted in history (an anti-gnostic notion if there ever was one) and “has revealed himself to everyone throughout history” (103). Furthermore, God uses history as the Pedagogue — the One who has progressively revealed himself to humans and who step by step prepared humans for Christ. There are three stages in this revelation: Creation (the Word is at work in creation and incarnation), Law (as the work of God to curb sinfulness), and Recapitulation (incarnation recapitulation all of human experience).
Four ideas flow from this for world religions:
1. God has universally revealed himself to all humans. In different ways, in different degrees, but revelation nonetheless. (Here is a breath of fresh air for those who are troubled deeply by a radical form of exclusivism.) God revealed himself to the Jews through the prophets and to Gentiles through creation. The pedagogic work of God is the work of Christ, the Word. Are religions a part of the pedagogic work for some?
2. There were “righteous pagans.” These are those who “by the Word were given light sufficient to hope for Christ” (110). People are judged on the basis of what they know.
3. Because God has revealed himself to all in various ways, “there is no salvation apart from accepting the gospel” (112), even if it means “postmortem contact with the gospel” (112-113). God, therefore, words ‘around’ the religions.
4. There is a certain historical relativism in Irenaeus — not a truth relativism but a historical one. That is, “God presents, by the Son, the truth of his Son in ways that are conditioned by historical circumstances” (113).