Back to the thread's topic, here are some of the most relevant texts on the biblical attitude to other faiths:
Just as God brought Israel into Palestine, He also brought the Philistines and Arameans to Palestine from distant lands. All these ethnic groups are led by God, even though they may not recognize it:
"Are you not like the Ethiopians to me, O people of Israel? says the Lord. Did I not bring Israel up from the land of Egypt and the Philistines from Caphtor (Crete), and the Arameans (Arabs) from Kir (a place in Iraq territory)?"
Paul concedes the possibility of being saved, apart from formal profession of faith in Christ: e. g.G
"To those who by patiently seeking to do good seek for glory and honor and immortality, He will give eternal life...When Gentiles who do not possess the Law do instinctively what the Law requires, these...are a Law to themselves. They show that what the Law requires is written on their hearts, to which their own conscience also bears witness (Romans 2:9,14)."
Paul seems to imply that pagans in the pre-Gospel era are not held accountable for their lack of faith in Him and hence need not be damned:
Paul: "God has overlooked the times of human ignorance, but now He commands all people everywhere to repent (Acts 17:30)."
"Sin is not imputed where there is no Law (Romans 4:15)."
What this means is that Christians must preach the Gospel everywhere, but must leave the final verdict on who is and is not saved to God.
Jesus seems to imply a non-exclusivistic big tent when He says: "Whoever is not against us is for us (Mark 9:40)."