John 13:8

He riseth from supper, and laid aside his garments; and took a towel, and girded himself.
After that he poureth water into a bason, and began to wash the disciples' feet, and to wipe them with the towel wherewith he was girded...
Peter saith unto him, Thou shalt never wash my feet. Jesus answered him, If I wash thee not, thou has no part with me.

I take it from this passage that unless we allow Christ to cleanse us and to serve us, we have no part with Him, and are not His. To be a Christian is to be continually asking for and relying upon God's help, more commonly called grace. In effect, we have all gone overboard and are walking on the water to Him; we are not sustained by our own power, but by His, according to our faith in Him. If we forget this, or if we start to worry about the wind and the waves, we will sink. The Lord allows this so that we can remember and learn by what power we have been brought hither.