This is the fourth question in the Westminster Shorter Catechism and the seventh question in the Larger Catechism.  You can quickly see why the Larger is longer than the Shorter.

The longer answer, in the Larger Catechism, is footnoted extensively with Bible proof texts, and goes like this:

God is a spirit, in and of himself

• Infinite in being, glory, blessedness, and perfection
• All sufficient
• Eternal
• Unchangeable
• Incomprehensible
• Every where present
• Almighty
• Knowing all things
• Most holy
• Most just
• Most merciful and gracious
• Long-suffering
• Abundant in goodness and truth.

This is another example of the Big God that the Westminster authors knew very well.