This is the sixth post in a unit on salvation in my ongoing series, theology in bullet points. The first section had to do with God and Creation, and I have also finished units on Christology and Atonement.
God fills us with his Holy Spirit as a seal of ownership on us.
1. We can repent, by the power of the Spirit. We have have faith, by the power of the Holy Spirit. God can forgive us. God can declare us cleared of all charges in his divine court and justified. We can even get baptized in water.
But the most fundamental feature of "conversion," of crossing from death to life. The "without which it hasn't happened" (sine qua non) is the Holy Spirit. "Anyone who does not have the Spirit of Christ does not belong to him" (Rom. 8:9). "Repent, and be baptized every one of you in the name of Jesus Christ so that your sins may be forgiven; and you will receive the gift of the Holy Spirit" (Acts 2:38).
The Holy Spirit is God's brand on us, his seal of ownership. "It is God who... has anointed us, by putting his seal on us and giving us his Spirit in our hearts as a first installment. " (2 Cor. 1:21-22). The Holy Spirit is the "earnest" of our inheritance, which is both a guarantee of what is to come but also a down payment of it (2 Cor. 5:5; Eph. 1:13-14).
In the book of Acts, the Holy Spirit is the crucial factor in salvation. In Samaria, there is a problem because individuals have been baptized but not received the Spirit. Peter and John come up to solve this problem (Acts 8:14-17). In Acts 10:44-48, no further discussion is needed about the possibility that uncircumcised Gentiles can be saved after Cornelius and his soldiers receive the Holy Spirit. They immediately assume baptism is the next step, even though the cleansing of their hearts is already accomplished (Acts 15:9).
At Ephesus, Paul does not consider baptism in the manner of John the Baptist to be effective, because they did not receive the Holy Spirit with it (Acts 19:2). He lays hands on them after they are baptized in Jesus' name, and they receive the Holy Spirit.
In all these instances, the Holy Spirit is the most crucial element in "conversion," inclusion in the people of God, of salvation. Repentance is not always mentioned. Faith can surely be assumed in the vast majority of cases, but is not always mentioned.  Baptism can be after the fact. Justification is of almost no interest to Acts. But no one is part of God's people unless they receive the Holy Spirit. 
2. Justification and sanctification have traditionally been considered twin sides to conversion to Christ. Justification has been considered the legal side of conversion. We are declared "not guilty" in the divine court. We are considered to be righteous in God's eyes and the divine court. Righteousness is imputed to us as a legal fiction--we are not really righteous, but we are considered such in the eyes of the law. 
Sanctification is a term that has then been used to refer to the actual righteousness that God imparts to us. He actually makes us righteous through the Holy Spirit that he imparts to us. The Wesleyan tradition sometimes calls this event, "initial" sanctification. God purifies us of our sins and positively makes us righteous. In Wesley's view, the person who is born to new life by the Spirit may still struggle against sin, but should still be able to make the right choices when faced with temptation because the Holy Spirit is within us.
There is nothing wrong with this theology, although it is probably not what sanctification terminology means in the New Testament. To "sanctify" (hagiazo) means to make holy (hagios). To be holy means, in its oldest meaning, to set something apart as God's. It is hard for us in our Western paradigms to make sense of this idea. Perhaps you might think of magnetizing something or touching something that belongs to someone who gets very upset when you touch his or her stuff.  You take off your shoes when you are on holy ground (e.g., Exod. 3:5). You treat the holy with great reverence and respect--it belongs to God.
Yet "to sanctify" also can have a sense of purifying something. For example, in Hebrews, to be sanctified has a primary sense of being cleansed of sins (e.g., Heb. 10:10).  Believers are set apart as belonging to God (2 Thess. 2:13) and, accordingly, they are not only purified from sin but must remain pure and undefiled (e.g., 1 Thess. 4:3). 
3. We might ask, given our scientific worldview, what exactly or literally it might mean to be filled with the Holy Spirit. In Paul's worldview, he may very well have pictured literal wind of a sort within us, a little bit of heavenly substance. After all, for the Spirit to be a down payment, we might picture a little bit of what is to come inside us. Hebrews 6:4 speaks of "tasting" the heavenly gift.
With the clarity of Christian theology, we have to believe that the Spirit has always been everywhere present. The difference is thus more on what we experience than on the presence of the Spirit. The person filled with the Spirit is the person empowered to do God's will. Receiving or being filled or being baptized is thus language about what we experience than any real change in God's presence, which was always there to begin with.
It is the Holy Spirit that makes it possible for us to love each other, the essence of righteousness. It is the Holy Spirit that empowers us to make the right choices, to choose to do the right thing (in mind and body) rather than the wrong thing. This power in our thoughts and actions is God's gift to us, the stamp on our lives that shows we are his.
Those who have faith in the atoning death of Jesus and put their faith in him as Lord experience the power of the Holy Spirit over their lives. They are "filled" with the Spirit.
Next Sunday: S7. God adopts us as his children and we are born to new life.
 It is possible to be filled with the Holy Spirit without exercising faith, as happens to John the Baptist in Luke 1:15. John is filled with the Spirit before he is even born.
 Most consider "receiving," "being filled with," and "being baptized by" the Holy Spirit to be more or less synonymous expressions in Acts, all of which relate to conversion. See James D. G. Dunn, Baptism in the Holy Spirit: A Re-examination of the New Testament on the Gift of the Spirit (Louisville: Westminster John Knox, 1977).
 It is important for the Calvinist tradition to say that Christ's righteousness is considered our righteousness or that when God looks at us, he now sees Christ. This is a nice picture, but it is nowhere stated in Scripture. The meaning of 2 Corinthians 5:21 probably was not about a transfer of sin and righteousness. It is a poetic expression that might be more literally unpacked something like the following: God made Jesus, who did not have any sin, to become a sin offering so that we might demonstrate the righteousness of God. See N. T. Wright, "On Becoming in the Righteousness of God," Pauline Theology, vol. 2, ed by David M. Hay (Minneapolis: Fortress, 1993), 200-8.
Having said that, it is hard not to hear a poetic play in 2 Corinthians 5:21, a poetic double entendre--Christ goes from righteous to sin. We go from sin to righteous. Nevertheless, while Paul does in at least one instance allude to a transfer of our miasma, our pollution and curse to Christ (Gal. 3:13), Paul never clearly speaks of the transfer of Christ's righteousness to us.
 I am not here emphasizing the angry part of this analogy, but the way a person engages something that is holy, namely, with great respect and caution.
 This is also more or less what Hebrews means when it speaks of "perfecting" a person who trusts in Christ's sacrifice.
 This language seems especially appropriate in the area of sexual defilement, where Paul retains traces of Levitical purity in his worldview.