Is the Holy Spirit really God?

Yes, according to historic Christian doctrine—the Holy Spirit is understood as the third person of the Trinity, fully God, not a lesser force or an impersonal power sent out by God.

In Acts, Peter tells a man named Ananias that lying to the Holy Spirit is the same as lying "to God"—the two are treated as directly, explicitly identical. The Holy Spirit is also described with attributes belonging only to God, such as knowing all things and being present everywhere at once, and Paul describes believers' bodies as a "temple" where God's own Spirit lives.

This doctrine was formally clarified at the Council of Constantinople in 381, extending the earlier Nicene Creed to explicitly affirm the Spirit is "worshiped and glorified" together with the Father and Son. One major, still-unresolved disagreement remains between Eastern and Western Christianity: whether the Spirit "proceeds" from the Father alone, or from the Father and the Son together—a real difference significant enough to have contributed to the historic 1054 split between the Catholic and Orthodox churches.