How do we know the necessity of washing in Jesus' blood?

In the narrative of John 13, Jesus tells Peter, "If I wash thee not, thou hast no part with me" (John 13:8). This indicates that beyond the physical act of washing feet, Jesus refers to the essential cleansing that comes from His sacrificial death. To be in fellowship with Christ, one must be spiritually cleansed from sin, which is accomplished only through His redemptive work on the cross. Hence, the washing symbolizes not just physical cleanliness but a deeper, necessary washing from sin through faith in Jesus Christ's atoning sacrifice.