Salvation is God's work
"Salvation belongs to the Lord" (Psalm 3:8).
When someone appeals to God and seeks forgiveness in Jesus, his sins are removed, he is cleansed, his relationship with God is restored, and he is made a new creature (2 Cor. 5:17). All of this is the work of God, not man.
The Bible has a phrase that describes the non-Christian. It is 'natural man'. In 1 Cor. 2:14 Paul says, "But a natural man does not accept the things of the Spirit of God; for they are foolishness to him, and he cannot understand them, because they are spiritually appraised."
Our human condition can be compared to a drop of poison in a glass of water: all the water is poisoned but it is not as bad as it could be. The water is incapable of being good. We, too, are incapable of really being good.
When Jesus' disciples asked Him who can be saved, He replied, "With men this is impossible, but with God all things are possible" (Matt. 19:26). That is why salvation rests in God alone by grace through faith (Eph. 2:8-9).
About now you are probably wondering what this has to do with witnessing. Why do you need to know all this? I am glad you asked. It is helpful to know because you must realize it is God who saves people. Specifically, it is the Holy Spirit who convicts the sinner of sin -- not you. "And He [the Holy Spirit], when He comes, will convict the world concerning sin, and righteousness, and judgment" (John 16:8).
Remember, the gospel is preached after sin is made known. Because the sinner cannot come to God on his own, he must be convicted of his sin, and thus be made aware of his need for salvation. The conviction of sin is beyond our control. It is the work of the Holy Spirit (John 16:8).
Prayer is essential in witnessing
It is, then, vital that you pray and request God to convict as well as save. Prayer is an essential part of witnessing. When you witness you must pray. Then you are free to spread the gospel as effectively as you want and to trust God to give the increase (1 Cor. 3:6-7). Ask Him to send the Holy Spirit; ask Him to convict the world of sin. The work of the Holy Spirit is essential in salvation.
The Holy Spirit
The Holy Spirit works in two types of people during witnessing: the saved and the unsaved. In the saved, He dwells within (Rom. 8:11), teaches (John 14:26), anoints (1 John 2:27), guides (John 16:13), and sanctifies (1 Pet. 1:2). Without the Holy Spirit we would be like ships without rudders, unable to live as Christians and certainly unable to witness effectively.
In the unsaved, He convicts of sin (John 16:8). Sinners come to Jesus to have their sins cleansed. They do this after they discover their guilt before God. This too is the work of the Holy Spirit.
Since the natural man is separated from God because of his sin (Isaiah 59:2), the Holy Spirit uses the Scriptures (that you quote) to convict him of his sinfulness, convince him of his need for salvation, and convert him through the Word. When a natural man (or woman) is aware of his sinful condition then the gospel message of deliverance from sin is preached and becomes effectual.
Sin
Sin does two things: it offends God and it kills man. How? It offends God because it is His law we break. It kills us because of the nature of Law. Have you ever heard of a law without a punishment? A law without a punishment is only a slogan. Since God is just and laws have punishments, then God must punish the lawbreaker. But that is not the end of the story. God is also merciful and gracious. His justice fell upon Himself -- on the cross. His mercy falls upon us -- by grace through faith.
Justice, Mercy, and Grace
Imbedded in the relationship of Law and Gospel are the concepts of justice, mercy, and grace. One of the best ways you can show the difference between them is to use illustrations that show their differences and relationships. For example, Justice is getting what we deserve. Mercy is not getting what we deserve. Grace is getting what we don't deserve.
Let's suppose you have a bicycle and I want it. So, one night I sneak over to your house and steal it. You catch me and I go to jail. (Jail would be where I "pay" for my crime of breaking the law.) The penalty is met and that is justice. I get what I deserve.
Let's change it a little. I sneak over to your house and steal your bike. You catch me. But you don't send me to jail. Instead, you tell me to forget about it. The penalty, jail, is not met. That is mercy. I did not get what I deserved.
One more change. I sneak over to your house and steal your bike. You catch me. You don't send me to jail. In fact, you give me the bike plus a hundred dollars. That is grace. The penalty is met (by you paying the 'damages') and I was given what I did not deserve (the bike and money).
Justice, which demands payment, does not meet the requirement of mercy, which seeks forgiveness. Mercy does not meet the requirement of justice. Grace meets both.
The Lamp Analogy1
Let's say I am at your house or apartment with my wife. We are talking about church and in my zeal I accidentally knock over your lamp. Now, this lamp is special. A dear friend gave it to you and it has great sentimental value, and besides, you need a light in your room. After a moment or two you realize that the damage is done and decide to forgive. You say to me, "That is alright, Matt. I forgive you for breaking the lamp, but give me ten dollars."
Is asking for ten dollars after you've just forgiven me, true forgiveness? Certainly not! When God forgives our sins, He says He will remember them no more (Jer. 31:34). Forgive and forget are similar in spelling and similar in meaning. If you forgive me can you demand payment from the one forgiven? No, because a forgiven debt does not exist.
Let's say that instead of asking me for ten dollars you turn to my wife and say, "Matt broke my lamp. You give me ten dollars for it."
I ask you again. Is that true forgiveness? No. You are simply transferring the debt to someone who was not involved in the original offense.
But, we have a problem. The lamp needs to be replaced. In true forgiveness, then, who pays for its replacement? (Think about this a bit before you go on to read the answer.) Who pays? You do! You're the only one left. Remember, if you've forgiven me the debt, how can you demand payment?
Now, who was my offense against? You. Who forgives? You do. Who pays? You do.
When we sin, who do we sin against? God. Who forgives? God. Who pays? God! Did you get that? God pays! How does He do that? Simple. 2000 years ago on a hill outside the city of Jerusalem He bore our sins in His body and died on the cross (1 Pet. 2:24). He took our punishment: "Surely our griefs He Himself bore, and our sorrows He carried... He was pierced through for our transgressions, He was crushed for our iniquities; the chastening for our well-being fell upon Him..." (Isaiah 53:4-5).
God is just. God is merciful. God is gracious. In the justice of God, He took our place. In the mercy of God we don't get punished. In the grace of God, He gives us eternal life.
Even though we are unworthy of salvation, even though we are unworthy of God's love, even though we are unworthy of mercy, even though we are worthy of wrath, God saved us. He did so not because of who we are, but because of who He is, not because of what we do, but because of what He did. God is love (1 John 4:16). God is holy (1 Peter 1:16). God is good (Psalm 34:8). We could never fathom the depths of His purity and kindness (Rom. 11:33). We could never, through our own efforts, attain Him. There is only one thing left for us. We must worship Him, love Him, and serve Him. He alone is worthy. Blessed be the name of the Lord.
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1. I do not remember where I read the lamp analogy, but I have been using it for many years. Someone contacted me and mentioned that it was used by Josh McDowell. If that is the case then he received the credit for the analogy.