How can a person that is painfully shy witness for Christ as bold as a lion?
The gospel comes with its own power. In other words, there is power in the Gospel, because there is power in the Word. The Apostle Paul, in referring to the gospel, wrote, “For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek” (Rom 1:16). Note that Paul said he is not ashamed of the gospel of Jesus Christ because it has the power of God in the message. How many of us look at ourselves as inadequate to share Christ and yet don’t realize that the gospel comes with its own power? Paul also told the Corinthian Church that “the word of the cross is folly to those who are perishing, but to us who are being saved it is the power of God” (1 Cor 1:18), so the gospel has the power to save. We only have to release it by sharing it. If we tell others about Christ, we can trust God with the results. We cannot save anyone, but God will use us as a means to save others when we share the gospel and let the power of God take over.
Ashamed of Christ?
Earlier we read that Paul was “not ashamed of the gospel,” but how many of us are? At one time, I was, but Jesus Christ tells us plainly that “whoever is ashamed of me and of my words, of him will the Son of Man be ashamed when he comes in his glory and the glory of the Father and of the holy angels” (Luke 9:26). If you never share Christ and deny Him by denying speaking about Him, then you are ashamed of Christ, and that does not end well for those who do this. Why? It’s because “the one who denies me before men will be denied before the angels of God” (Luke 12:9). This would be like bringing a date to a party and then denying he or she is your date, and then, even denying that you knew them. Wouldn’t that humiliate and anger your date? Of course it would, and it might be the last date you had with that person, so imagine how Jesus feels when we deny Him by our silence and we are ashamed to speak about Him.
Fear of Man
Three of the most frightening things we do as humans is: pubic speaking, having a fear of heights, and having a fear of insects (spiders in particular), but for believers, I would put the number one fear as sharing Christ. Fearing people’s rejection of Christ is the reason most people don’t witness, but the fact is, “The fear of man lays a snare” (Prov 29:25a), but our fear’s in the wrong place. Jesus again tells us to “not fear those who kill the body, and after that have nothing more that they can do” (Luke 12:4). They can persecute you, call you names, and in some places around the world, even kill you, but Jesus reassures us that, “after that have nothing more that they can do,” so the One to fear is God and Him alone. We can actually deny Christ by not speaking about Christ. By our silence, we deny Jesus because we fail to speak up for Him, and that’s another serious issue as Jesus says that “whoever denies me before men, I also will deny before my Father who is in heaven” (Matt 10:33). If we are ashamed of Christ in this lifetime and deny Him before others in our lives, then He will be ashamed of us and deny us before the Father, meaning we will not enter into the kingdom. Why do we fear man over an omnipotent God? Maybe it’s because we don’t like to look bad in front of others or be embarrassed when people reject us and the message of the gospel, but by doing so, we show we fear man more than we fear God.
How do you conquer fear? You go out and get scared a lot of times. What I mean is, if you are fearful of witnessing for Christ (most of us are), then practice with someone in your family or another Christian. And don’t think that their being saved depends on you because it doesn’t…God alone saves, although we have the great privilege to participate in the Great Commission. Witnessing gets easier over time, but never really being “easy.” It’s better if you have someone with you who can pray while you share Christ. Maybe that’s why Jesus sent them out two by two, but one way you can destroy the shyness you have (or really, it’s fear!) in sharing Christ is to think about the fact that God saves them by His Word and by His Spirit. Think of the person’s eternal state if you don’t witness to them. I don’t think we’ll be in heaven thinking, “I wished I hadn’t embarrassed myself by witnessing to my aunt Kathy” but more likely, “Why didn’t I share Christ with my aunt, uncle, nephew, and neighbor!?” There is no evangelism in heaven, because by then, it’s too late, but as long as you’re drawing a breath, God can use you. You must pray to get over your fear of witnessing and think about these people being separated from God for all eternity. That should motivate us to be more consistent and effective witnesses for Christ.
If you have never, ever shared Christ with anyone, then you must be ashamed of Him or you are fearful of man. You must fear the rejection of man more than the fear of God. God commands us all to go into all the world to make disciples of all nations, teaching them the same things that Jesus taught the disciples (Matt 28:18-20), but if you fear speaking to people about Jesus, then you won’t go into all the world. In fact, you might not even go next door. Sharing Christ is not easy, but it is commanded of all believers. And if we don’t share Christ, who will? Remember, someone shared Christ with you, so how can you and I possibly be too shy, fearful, or ashamed to share the good news of Jesus Christ?
Here is some related reading for you: How to Share the Gospel with Family 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.