“The multiplying witness of the church has two elements to it, according to this passage. The first is proclamation of the message (John 17:20) which is to be believed (17:20, 21, 23).
The second is the public demonstration of the unity for which Jesus prays (17:21, 23), calling to mind the purpose of the ‘new commandment’: ‘All men will know that you are my disciples if you love one another’ (13:35).
Both aspects of our witness are essential. The truth of the gospel, announced without the demonstration of the power of the gospel in transformed and loving lives, is arid. It may be beautiful in the way that the badlands can be beautiful; but not much grows there.
On the other hand, the demonstration of love within a believing community does not by itself proclaim the source or cause of that love. Attractive in its own right, like a luxuriant south sea island, nevertheless such love does not call forth disciplined obedience or informed belief, and cannot of itself call others to true faith. It is merely a place to rest.
The multiplying witness Jesus has in mind is both propositional and exemplary, both confessional and demonstrative. It is a witness of word and of love.”
–D.A. Carson, The Farewell Discourse and Final Prayer of Jesus: An Exposition of John 14–17 (Grand Rapids, MI: Baker Book House, 1988), 200. Carson is commenting on John 17.