Do you have any concerns regarding the content of the Speratus confessional hymn apart from baptism?
Actually yes, there are other problems with the hymn: The phrase, “Who did for all the world atone,” is incorrect, as Christ only made atonement for the elect. “From sin our flesh could not abstain,” which could point to someone saying they are not responsible for their actions (i.e. My flesh made me do it and Adam is responsible). With all these errors I would not be doctrinally correct in signing this hymn.
Some observations:
1. The line “Who did for all the world atone," is translated, "Who hath for us deliverance won -" in the 1905 LCMS hymnal. The original German is "Der hat g'nug für uns all' getan,". Whether later translations were an attempt to promote universal atonement, I'll leave to others to decide. That said, I agree that Jesus Christ "did for all the world atone."
2. “From sin our flesh could not abstain,” teaches bondage of the will. Nowhere in the hymn does Speratus deny personal responsibility for sin. "What God doth in His law demand, No man to Him can render: Before this Judge all guilty stand; His Law speaks curse in thunder."
3. I commend your demand for doctrinal purity in hymnology. Confessional hymns defend the faith once delivered unto the saints.
4. The doctrinal purity of Speratus is irrevelant to the issue as to whether "Salvation Unto Us is Come" is doctrinally pure. What is important is that the hymn is not sung to give a false testimony of unity when there are divisions in doctrine. So your questions regarding Speratus personal views on baptism are not without merit.