That's a question only someone who doesn't read the Bible would ask. “To the Jews I became as a Jew, in order to win Jews. To those under the law I became as one under the law (though not being myself under the law) that I might win those under the law.” — 1 Corinthians 9:20 Christ being our Passover Lamb doesn't mean that the historical Passover suddenly became meaningless. The Passover was a real act of deliverance that God worked for Israel, and it also typologically pointed forward to Christ. There's nothing inherently wrong with Jewish Christians observing Passover, provided they don't treat the ceremony as necessary for salvat or as though Christ's sacrifice were insufficient. Christ is the fulfillment of what the Passover signified; that doesn't erase the historical event or make remembering it sinful. Paul continued to participate in Jewish customs and arranged his travels around Jewish feast days (Acts 18:21; Acts 20:6, 16). The issue isn't whether the feast is observed, but what significance is assigned to it.