If we have experienced the gospel as Good News, we will not be able to keep it to ourselves. It's in the Bible, II Kings 7:9, TLB. "Finally they said to each other, 'This isn't right. This is wonderful news, and we aren't sharing it with anyone!"
Jesus' command compels us to witness. It's in the Bible, Matthew 28:18-20, TLB. "He told His disciples, 'I have been given all authority in heaven and earth. Therefore go and make disciples in all the nations, baptizing them into the name of the Father and of the Son and of the Holy Spirit, and then teach these new disciples to obey all the commands I have given you; and be sure of this—that I am with you always, even to the end of the world."
Live unashamedly for Christ and be a light to the world. It's in the Bible, Matthew 5:16, NKJV. "Let your light so shine before men, that they may see your good works and glorify your Father in heaven."
Christians today may not be eyewitnesses, but we are faith witnesses to Christ's power in our life. It's in the Bible, Acts 1:8, NIV. "But you will receive power when the Holy Spirit comes on you; and you will be My witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth."