Steve Ray has a list of 35 loaded Questions for "Bible Christians" (quotation marks his)(link to the whole list). This is number 33/35. I'm trying to provide the answers in a common format, for easy reference.
33) Jesus said that the unity of Christians would be objective evidence to the world that He had been sent by God (John 17:20-23). How can the world see an invisible "unity" that exists only in the hearts of believers?
The world sees the invisible unity through the visible manifestations of invisible love that believers have for one another.
John 13:35 By this shall all men know that ye are my disciples, if ye have love one to another.
1) The ungodly world doesn't recognize the Spirit of Truth,
John 14:17 Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you.
or the sons of God:
1 John 3:1 Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
2) Believers are directly taught by God to love one another.
1 Thessalonians 4:9 But as touching brotherly love ye need not that I write unto you: for ye yourselves are taught of God to love one another.
3) I've previously demonstrated that Rome's apologists appeal wrongly to John 17 here: (link).