Through the indwelling of the Holy Spirit, the Father and the Son come to make their home within our hearts (John 14:15-24). Anyone who does not have the Spirit of Christ does not belong to Him (Romans 8:9). Williams correctly understands the implications of these texts that divide the world into two groups (those who are Christian, and those who are not) rather than three categories (those who are not Christian, those who are Christian but not filled with the Spirit, and those who are both Christian and Spirit-filled).
I find the title misleading in that it implies that being filled with the Spirit is something that Christians might or might not decide to do. However, Williams provides a study guide that will prove helpful to groups looking for an introduction to the present ministry of the Spirit within their lives.