J. Mack Stiles, Marks of the Messenger: Knowing, Living and Speaking the Gospel (Downers Grove: IVP, 2010), 103–9:

Loving Other Christians and Evangelism

On the eve of his death Jesus said, “A new command I give you: Love one another” (John 13:34). This is by far the most important command for healthy evangelism. We are to love the church. . . .

“A new command I give you: Love one another. As I have loved you, so you must love one another. By this all men will know that you are my disciples, if you love one another” (John 13:34–35). . . .

[T]his love must be seen by the world, not just felt by believers. . . .

How odd, one might say, that the best way to demonstrate that Jesus is from the Father and that we are his followers is not through method or technique or apologetics. It’s through loving, unified community among believers. . . .

So, the method of evangelism that Jesus institutes is unity and love among believers. Yet how often I meet would-be evangelists who don’t love the church; to paraphrase, they love the church in general, but no church specifically.

How do we wear the mark of love before the watching world? How do we make the shape of our community cruciform? These are radical questions, and they require a radical response.

What follows is a list of radical, mind-blowing, evangelism-producing principles that, if practiced, will change the world. You have fair warning—these are countercultural, even bizarre to the watching world, but they demonstrate love for one another.

16 Ways to Demonstrate Love and Unity in the Church

1. Attend a church that takes the gospel seriously (Hebrews 10:25). Treat form as secondary, the gospel as primary. . . .
2. Become an actual member of a church. I’m serious; membership shows your loving commitment to one another. . . . And just think, the less cool the church the more opportunity to demonstrate real love!
3. Read C. J. Mahaney’s book Humility once a year. . . . Mahaney gives us practical tools for demonstrating love and unity in the context of community.
4. Turn down jobs that might take you away from church even if they pay more.
5. Arrange family vacations around your church’s schedule. Or better yet, take your family on a short-term mission trip with other members instead of a family vacation. This will blow people’s minds.
6. If your church doesn’t have a church covenant, think about developing one that expresses your love for each other.
7. Move to a house closer to the church [building] and use your house as a place of hospitality (Romans 12:13).
8. Practice church discipline. . . .
9. Respect, even revere, the authority in the church (1 Thessalonians 5:12–13).
10. Turn heads—really practice the biblical teachings of giving and receiving forgiveness. . . .
11. Take care of people who are in need physically in your congregation (Romans 12:13).
12. Pray for each other (Ephesians 6:18). Don’t just say you’ll pray. Actually put into place some ways to pray for each and every member.
13. Sympathize with other believers (Romans 12:15). Check a critical spirit.
14. Focus on caring for one another spiritually by discipling one another (Galatians 6:1–2). . . .
15. Share your faith together (Philippians 1:27). . . .
16. Read Mark Dever’s 9 Marks of a Healthy Church.

. . . [T]his kind of radical love leads to radical evangelism.

To be a healthy evangelist means to love brothers and sisters.

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I’ve heard Don Carson describe Stiles’s book several times as the best book on evangelism he’s read.

For more info (e.g., endorsements by Mark Dever, D. A. Carson, C. J. Mahaney, and Thabiti Anyabwile), see here.