What does it mean that God is spirit?

God is a Spirit.

As God is a Spirit, and you are His child, then it follows without any reserve whatever that you, too, are spirit because you are of Him.

Christ goes in front—you merely follow after. 

It is not so difficult as it sounds—Christ, and that long, long army, clothed in the beauty of the spirit, they show you the way, and once your eyes are fixed on the light, shadow and gloom are lost in the radiance, and you, as you progress, are able to act as light to those who are behind.

—Zodiac, the Christ Messenger