Now this is no problem within Orthodox Christianity, where every set of prayers includes the prayer beginning, "O Heavenly King, the Comforter, the Spirit of Truth Who are everywhere and fill all things..." the Spirit of Truth makes His earthly home in the Church, but is never confined to her; He is everywhere. The Orthodox Church has no monopoly on Truth. She has the fulless of Truth (Christ), but it's impossible for anyone, created in the Image of God, to be completely wrong about everything all the time. Just because adherents of other religions are handicapped by errors doesn't mean there is no value in those religions; of course there can be! (I do not say there necessarily always is.) In Judaism for example, Christ is all over every page of their Scripture.
As for the fate of the non-Orthodox, we leave that confidently in the hands of the all-righteous One, Whom we call all-righteous precisely because He is all-loving. We do not feel the need to define who will be in and who will be out, and suspect the attempt may sometimes be presumptuous. We understand that He Who promises to prune dead branches from His Church on the Last Day also has the power, the right, and the know-how to graft in new branches on the Last Day.
So does salvation necessarily have to be in Christ, as Dr. Knitter wonders? Of course it does! Because Christ Himself is salvation, is the very definition of it: Christ is heaven, and heaven is Christ. Forget the pearly gates and golden streets, the mansions and crowns, which apart from Him would be useless, empty, joyless! He is what makes heaven to be heaven. Communion with Him, and with each other in Him, is heaven. Living His life together, sharing with Him in the very inner life of the Holy Trinity, is heaven, is joy, is love, is blessedness, is our ultimate fulfillment, meaning, and bliss. Starting now.