I have other sheep that do not belong to this fold. I must bring them also, and they will listen to my voice. So there will be one flock, one shepherd. - John 10:16
There is yet more light and truth to break forth from God's Word. - John Robinson (Puritan Preacher, 1620).
According to Dr. Byron E. Shafer in a sermon he preached at Rutgers Presbyterian Church in NYC back in 2005: "Robinson believed that, in principle, the Holy Spirit can lead us to find truths in Scripture that no Christian group or generation has previously seen or understood." And goes on to say that "... I believe that when Christians today read in John 10:16-"I have other sheep that do not belong to this fold"-the Holy Spirit is seeking to lead us beyond the view that there is no salvation outside of Christianity-beyond that view to the understanding that God does have other sheep and that Christianity is not God's only sheepfold."
His sermon is ultimately less than affirming of other faiths or at best reaches no conclusion about what those "sheep that do not belong to this fold" mean. But reading between his lines, I think, he sees that other faiths have much to teach us and that God has chosen those other faiths for reasons unknowable to impart wisdom to us. You can read it all here if you'd like.
Today, my doubts are as ever. But I take comfort in the idea that there are "sheep that do not belong to this fold" (I may well be one of them) but that nonetheless Jesus will bring them together in some way that is unfathomable. Sometimes more mystery makes it easier to believe. Three persons in One God. Many religions that will listen to Jesus' voice.
Despite my doubts, this bible verse centers me, and I await "more light and truth to break forth from God's word."