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.
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.