Just a little more biblical input, but what part does the Holy Spirit play in salvation?
On the night of the last supper, Jesus spoke about the "helper" that he would send to His disciples and we understand that the helper is the Holy Spirit.
7 Nevertheless I tell you the truth. It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you. 8 And when He has come, He will convict the world of sin, and of righteousness, and of judgment: 9 of sin, because they do not believe in Me; 10 of righteousness, because I go to My Father and you see Me no more; 11 of judgment, because the ruler of this world is judged. John 16:7-11
So after Jesus' death and resurrection the Holy Spirit was sent and will reside inside you for the asking, but under what conditions?
Well, Jesus said that He (the helper) will convict the world of sin, of righteousness, and of judgment. In 1 John we have a description of "the witness":
6 This is He who came by water and blood—Jesus Christ; not only by water, but by water and blood. And it is the Spirit who bears witness, because the Spirit is truth. 7 For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one. 8 And there are three that bear witness on earth: the Spirit, the water, and the blood; and these three agree as one.
So, while anyone can call you a sinner, or point at some specific sin in your life, or you can read about some sin in scripture that you may have committed, no one but the Holy Spirit can actually convict you of sin, actually call you guilty and convince your heart of that sin or that you are indeed a sinner.
How does He convince us that God's description of sin is something real and not made up? The Holy Spirit also convicts us of righteousness and Jesus says that this is because of His death and resurrection. In other words, it is God the Holy Spirit who confirms the truth of His word in raising Jesus Christ from the dead and in His ascension into Heaven. This is not something we naturally believe, even based on the testimony of other people. Do you believe everything anyone tells you, no matter how impossible it sounds, even if they swear it's true? The Holy Spirit is the witness of God that tells us that Jesus rose from the dead and returned to His father, the reason we have genuine belief without a doubt. If He speaks to us and convicts us of sin and is telling us (in our spirit) that Jesus is risen from the dead, and we are rejecting what He says, consider it a trick of the mind, a delusion, our imagination, etc., Then we are attributing His work of salvation as nothing, as air, the definition of blasphemy.
You can reject what people have to say about Jesus. The apostle Paul not only rejected the person of Jesus before his conversion on the road to Damascus, but actually participated in the arrest and murder of Christians. Paul was converted when He met the resurrected Christ, was struck blind, and then met with a Christian who had "the witness" within him, and who literally opened his eyes by the power of the Holy Spirit. If you reject the gospel when the Holy Spirit is convicting you of sin and of righteousness and judgment, there is no hope for you. If God can't convince you of the truth, no man will ever do so. The more controversial aspect of this encounter with God is the indwelling of the Holy Spirit. Some people believe that you can loose the Holy Spirit somehow, but this idea is not supported by scripture. He is a seal of redemption, a proof of adoption. A satisfaction of Jesus' promise to be with us and never leave us. I suspect many on these forums might dispute this, but the Bible addresses this very topic on multiple occasions, and you have to satisfy yourself with one important question. Who saves us? Is it God or is it us? I can tell you the evangelical point of view, but there is disagreement between the churches, because some believe that a priesthood is required to dispense grace as the Jewish priests did, while evangelicals say that it is genuine believers who are a sacred priesthood and that all genuine believers are saints, sanctified by the presence of God within us.