One of the things Christians argue about is the person of the Holy Spirit. It is my understanding that when you become a Christian you are baptised in the Spirit. I say this because the apostle Paul teaches us that all true believers have the Holy Spirit dwelling within them (Rom. 8:9-11) and he refers to Christians as all being baptised by one Spirit into one body (1 Cor. 12:13). Whereas in the Old Testament some of God's people had the Holy Spirit dwelling with them some of the time, the Day of Pentecost (Acts 2) marks the beginning of an era when all God's people are indwelt with the Holy Spirit all of the time. In the following clip Mark Driscoll explains what it means to be filled by the Spirit.
If you enjoyed that video you might like to watch this second video concerning the active role of the Holy Spirit in our lives.