The Holy Spirit is the active agent of the God-head in the salvation of the lost. Prior to salvation, the ministry of the Holy Spirit is to convict the sinner of his lost state. In John 16 Jesus explains the ministry of the Holy Spirit to the lost. Jesus stated that the Holy Spirit will reprove the world of sin. This means that the work of the Holy Spirit amongst unbelievers is to reveal their sin and cause them to recognize a need for salvation through Christ. The Spirit accomplishes this through a number of ways. One such way is when the Spirit of God quietly speaks to the heart of an unbeliever, pointing out short-comings and failures. This is one reason why the self esteem movement is so dangerous as it deceives people into believing that they are good in and of themselves and there is no need for God or salvation. Self esteem teaching becomes the antithesis of the work of the Holy Spirit in the life of an unbeliever.
Another way the Spirit works in the hearts and minds of unbelievers is through the lives of believers of whom the Spirit dwells. As we are tabernacles of the living God, we are indwelt by the Spirit of God and are used to convey the message of God in both word and deed. I Peter 1:1 demonstrates this in the life of a believing wife as she bears witness of her faith through her godly lifestyle. This verse points out that at wife through her actions will demonstrate God to her unbelieving husband and that will supplant the Word of God in his life when he does not read the Word. As believers we are called to bear witness to Christ by the power of the Holy Spirit. In Luke 12:11-12 Jesus tells the disciples that they need not worry about what to say when they are brought before rulers and questioned but that the Spirit would tell them what to say. We too must be in tune with the Spirit of God that we may be able to hear and speak the words He places in our hearts for the lost.
The work of the Spirit does not end when a person comes to salvation, the Spirit then indwells the believer and becomes a comforter, a guide and a counselor, bringing about the work of sanctification in our lives. Romans 8:29 tells us that we have been predestined to be transformed into the very image of Christ. That is the work of sanctification, the Spirit making us useful for kingdom work.