What are the seven spirits of God in the book of Revelation?
The book of Revelation is written in symbolic or figurative terms according to its author. "The Revelation of Jesus Christ, which God gave Him to show His servants--things which must shortly take place. And He sent and signified it by His angel to His servant John" (Revelation 1:1). The word translated here as "signified" is the Greek word semaino, which means to give a sign or a representation. Thus Revelation must be read with care because it contains representations of things and not direct statements.
In a sense, Revelation can be viewed as a series of pictures described in words. These pictures represent important ideas. Sometimes the key to these pictures are given within Revelation, but God also intended the book for Christians who would be familiar with the rest of the Bible. Even numbers have symbolic meaning in the text. Not in the idea of secret codes, but in representing ideas or concepts.
Seven is the number of days in a week, a value that has not physical correlation to the sun, moon, and stars. It is an idea that has always existed and the Bible shows its origins to be in the Creation week. "And on the seventh day God ended His work which He had done, and He rested on the seventh day from all His work which He had done" (Genesis 2:2). In Hebrew, the number seven and the word for oaths are very closely related. Thus the number seven represents something that is complete or perfect and it also represents something that has been promised or vowed.
The seven Spirits of God would be the complete or perfect Spirit of God -- the Holy Spirit. The Holy Spirit is our guarantee of salvation. "In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory" (Ephesians 1:13-14). Just as Jesus appears in Revelation as a lamb that was slain, the Holy Spirit appears as the seven Spirits of God.