Having defined the gospel as the message that proclaims the arrival of God's blessed kingdom and the means of citizenship in it by faith in Christ alone, we can now move on to the broader biblical teaching on what it means to proclaim this message to all creation. As we will see, all believers are called to bear witness to the truth of Jesus Christ.
First, we should consider the distinction between witnessing and evangelism. Essentially, witnessing is a broad category of which evangelism is a subset. Biblically speaking, the task of bearing witness is to make visible what is otherwise invisible to us. Our task as Christians is to show forth the reality of God's blessed reign. We are to bear witness to the invisible kingdom of God and make it visible by testifying to its existence in a variety of different ways.
Bearing witness to the reality of God's kingdom includes telling the story of Jesus and calling people to repent and trust in Him—evangelism—but that does not exhaust the ways we bear witness to our Lord's saving reign. For example, the love we show to other believers is a powerful testimony to the rule of Jesus our King. When we love one another in the church, we show that we belong to Christ, who pours His love into our hearts (John 13:34–35). Another way we witness to the kingdom of God is through our celebration of the Lord's Supper. Strictly speaking, the Lord's Supper is not an exercise in evangelism, though it visibly portrays some of the content we preach in evangelism. As often as we eat the bread and drink the cup, we show forth the Lord's death until He comes (1 Cor. 11:26).
We should note two things at this point. First, that we will be witnesses to Christ and His kingdom is not in doubt. Today's passage records Jesus' final words to His followers before His ascension, and He says, matter-of-factly, that those who serve Him will be His witnesses. We will either be good witnesses or poor witnesses, but we will certainly be witnesses. It is inherent to being a Christian.
Second, knowing the distinction between witnessing and evangelism keeps us from confusing the two. Giving our testimony of how God saved us and is blessing us is a good thing, but it is not evangelism. Living in holiness witnesses to the work of Christ's Spirit, but it is not evangelism. Evangelism takes place only when we share the message of Jesus' life, death, and resurrection and then call people to trust in Him alone.
Most of us have likely been told to "preach the gospel at all times; if necessary, use words." That's a well-intentioned but misleading phrase. We have not preached the gospel if all we have done is been kind to others. Kind acts adorn the gospel and show that the One whom we serve is conforming us to His image. But evangelism happens only when we proclaim the message of Jesus and call people to put their faith in Him.