How do we know the Sabbath is still relevant for Christians?

The Sabbath is indeed relevant but in a different context for Christians. The New Testament reveals that the Church does not observe the Sabbath in the same way as the Old Covenant. According to Colossians 2:16-17, believers are not to be judged regarding the observance of Sabbath days as these were shadows of things to come, with Christ being the substance. Thus, the relevance lies in understanding that Jesus is our eternal rest and that we are to live continually in a state of spiritual rest in Him, rather than adhering to a strict sabbatical law.