"God is a Spirit, and they that worship him must worship him in spirit and in truth." - John 4:24 (Audio)
Here are some sermons on John 4:24:
God is a Spirit, by Dr. David P. Murray
Acceptable Worship, by Rev. Colin Mercer
True Worship, by Rev John Greer
The Pattern for True Worship, by Rev. Stephen Hamilton
The Prerequisites of True Worship, by Rev. Stephen Hamilton
The Being of God, by Rev. Roger Higginson
God is a Spirit, by Dr. Jerry Bilkes
