Many thanks to my wife who pointed out this helpful snippet from her daily devotional (in the June issue of Tabletalk Magazine):
God is building us into His holy temple, but that does not mean we lack a role in the construction process. Unlike regeneration, sanctification is a cooperative work between the Holy Spirit and us (Phil. 2:12-13). Because of His work, we strive unto holiness in this life, and our efforts to mortify sin evidence that we are indwelt by the Spirit and truly belong to Jesus. We prove our election as we seek to live as holy people in this world.