Macedonianism is an Anti-Trinitarian heresy taught by a group of people known as the Pneumatomachi (Combators of the Holy Spirit) and was so named after Macedonius, who was Patriarch of Constantinople.
Macedonius and his followers were semi-Arian, and taught that though the Son was eternal, He was not of one essense (Latin: consubstanciales, Greek: homoousios) with the Father but of like essense (Greek: homoiousios) with the Father. They also taught that the Holy Spirit was not eternal and consubstantial with the Father and the Son but a creation of the Father and an action of the Son. Thus, the Macedonians denied that the Holy Spirit was a hypostasis, or person, of the Holy Trinity.
Many fathers wrote in opposition to Macedonianism, including Athanasius the Great, Basil the Great, Gregory of Nyssa, and Gregory Nazianzen. Macedonianism was condemned at the Second Ecumenical Council, which inserted the following words into the Nicene Creed:
"Καὶ εἰς τὸ Πνεῦμα τὸ Ἅγιον, τὸ κύριον, (καὶ) τὸ ζωοποιόν, τὸ ἐκ τοῦ πατρὸς ἐκπορευόμενον, τὸ σὺν πατρὶ καὶ υἱῷ συμπροσκυνούμενον καὶ συνδοξαζόμενον, τὸ λαλῆσαν διὰ τῶν προφητῶν."
Which translates as:
"And [I believe] in the Holy Spirit, the Lord, the Giver of life, Who proceedeth from the Father, Who with the Father and the Son together is worshipped and glorified, Who spake by the Prophets."
- Talberg, D. N., История Христианской Церкви (A History of the Christian Church), Moscow, Russia: St Tikhon's Orthodox Theological Institute.