Arianism

The Arian concept of the nature of God is that the Father is greater than the Son and that the Son had a beginning. This is what distinguishes Arianism from Trinitarianism.

"The main argument of the Arians was that Christ was a Son, and therefore was not eternal, but of a substance which had a beginning. Thus Arius, in his debate with Alexander, urged that: "If the Father begot the Son, he that was begotten had a beginning of existence. From this it is plain that once the son was not, and it follows of necessity that he had his subsistence out of nothing." - Historical Christology, Chapter V - Arianism and the Council of Nicea, by Father John A. Hardon.

The history behind Arianism and the controversy that arose during the time of Constantine and how he got involved is intriguing. You can read about it here: How the 4th Century Church Fathers Declared Jesus Equal to God. Interestingly, Constantine had a key role in the change of Arianism, the orthodox view at the time, to the Athanasian doctrine (previously unorthodox). This occurred at the Synod of Nicaea in A.D. 325. Some years later at the Synod of Constantinople (in A.D. 397) the Holy Spirit was added to the "co-equal, co-eternal, and co-substantive" phrasing and thus established the Trinity doctrine.

The Nicene Creed

In A.D. 325, emperor Constantine convened a council at the city of Nicaea to stop stop Christian squabbling over the nature of God and to establish once and for all, by majority vote, what Christians must believe to be saved. The debate over Arian versus Athanasian views on the nature of God were settle here. "The Nicene Creed" was the result and remains the official statement of Christian faith recited every Sunday in many Christian churches. Some Protestant churches use a simplified version known as the "Apostles' Creed." Both versions reject the idea of one God while claiming to embrace it. Simply put, the doctrine, and "mystery," of the holy trinity holds that while there is in fact only one God, "he" consists of three distinct, yet indivisible parts: "God the Father," "God the Son," and "God the Holy Spirit." If it made sense, there would be no need for faith to believe it. This is true of many religious doctrines. After Nicaea, the belief in only one God, instead of the triune God, became a heresy that could get you killed.

See also:

• The Arian Controversy with Alex Hall. Alex Hall discusses the history behind Arianism. Interestingly, all the information we have about Arianism is from secondary sources. All original sources were wiped out by those opposed to it.
• Understanding Jehovah's Witnesses.