John Calvin Commentary John 13:17

"If ye know these things, blessed are ye if ye do them." — John 13:17 (ASV)

If you know these things. He declares that they are happy, if they know and do these things; for knowledge is not entitled to be called true, unless it produces such an effect on believers as to lead them to conform themselves to their Head. On the contrary, it is a vain imagination, when we look upon Christ, and the things which belong to Christ, as separate from ourselves.

We may infer from this that, until a person has learned to yield to their fellow believers, they do not know if Christ is the Master. Since there is no one who performs their duty to fellow believers in all respects, and since there are many who are careless and sluggish in mutual service, this shows us that we are still at a great distance from the full light of faith.