The only reason someone would want another sure pledge is because they do not trust the first sure pledge. Surely it indicates a lack of trust in the One making the pledge, but trust in the second pledge-maker. ?The Marian prayer in the OP calls Mary "a sure pledge of my eternal salvation". Well, that is what the Bible calls the great Holy Spirit. Why would we ever need another "sure pledge" of our salvation than the great Holy Spirit Himself, Who is God? Can there be a better pledge than God almighty Himself? Why isn't God ever good enough for Catholics?
--Rich
"Esse quam videri"