Is it legalistic to keep the Sabbath Holy as a Christian? – No…but…
If you desire to keep the Law to honor Jesus Christ, the Father, the Holy Spirit and if you desire to keep the Sabbath ceremonial edict because the Holy Spirit given you as Guarantor of relationship with the Father by faith in Jesus (Ephesians 1:13-14) has moved you to honor the Sabbath…if these things bring you “peace” in your relationship with God, by all means, observe them…BUT…do not allow yourself by demonic influence to progressively believe that what your “doing” to honor the Father somehow makes you righteous and acceptable to the Father by supplementing the work of Jesus at Golgotha as atonement for your sin.
Beware of Jewish, Christian cultic, legalistic apostasy,
This is the eternal error of far too many “Messianic Jews” who live with one-foot in the Law and one-foot in Grace by Faith in Yeshua. They simply cannot bring themselves to trust in the SIMPLICITY of the Gospel message that God became flesh; entered Time through the womb of a virgin young woman; lived in perfect righteousness by fulfilling the moral requirements of the Law; then walked to the horrors of a Roman Crucifixion and shed His blood as atonement for our sin and access to that atonement granted by “faith alone.”
Be careful of the sin of the Galatian Jews that you don’t spurn the Gift of God accessed by faith, alone, for works of self-righteousness thereby making yourself unworthy of God’s grace in Jesus Christ-Yeshua (Ephesians 2:8-9). There is no peace with God when man seeks works of self-righteousness to supplement the eternally sufficient work of the Son, Jesus Christ, at Golgotha. Messiah’s atoning work upon the Cross is eternally sufficient to atone for your sin (John 14:6). Your works of righteousness SUBSEQUENT to faith in Jesus, alone, simply authenticate your faith as true but works can NEVER supplant saving faith as true faith is essential to establish covenant relationship with the Father (John 3:16-18).