What does John 17:21 mean?

Scripture often emphasizes how unity and love reflect a person's commitment to Christ (John 13:34–35; 1 John 4:20). It's noteworthy that when Jesus transitions from praying for Himself (John 17:1–5), to praying for the disciples (John 17:6–19), to praying for all Christians (John 17:20), the first thing He refers to is unity (Psalm 133:1).

Christian "love" is meant to distinguish believers from non-believers. The unbelieving world cannot see or experience the Holy Spirit (1 Corinthians 2:14). All they can see is how self-professed believers speak and act (Matthew 5:13–16). In this verse, Jesus points out this is how "the world" (John 17:16–18) is meant to be introduced to faith in Christ.

This is why those who say they are Christians must behave accordingly (1 Peter 2:12) and held accountable by other Christians (1 Corinthians 5:11). The need for unity does not mean tolerating blatant sin or false teaching (Galatians 1:8–9; 2 Timothy 3:16). However, the Bible is clear that those who love God will obey God (John 14:15), and the primary demonstration of that is a unifying love for other believers (John 17:23).

Jesus makes other comments implying that He is God and shares the divine nature with God the Father (John 5:22; 10:30; 14:9). In this specific context, His reference is to unity of purpose and intent. Jesus' actions, consistent with the words of God and the will of God, were proof that He was sent by God (John 14:11, 31). The same is meant to be true of Christian believers: that their actions and attitudes reflect a commitment to their Creator (Colossians 3:14).