Oh Arius, you Darnel!

But while his men were sleeping, his enemy came and sowed tares among the wheat, and went away.
Matthew 13:25 

Arianism was among the major issues discussed at the Council of Nicaea in 325. At that time there was no commonly adopted doctrine about anything in the Kingdom of God. Persecuted Christians did not have the chance to come together to talk about the content of their faith. The Scriptures were a gentle guide drawing the line between Judaism, Christianity and paganism but as we know, they raise as many questions as they answer.
Constantine I had learned to his dismay that bitter disputes existed among the Christians also about the most fundamental issues, such as Who is God. How could such a confused religion unite the people of the Empire under One God and one Caesar?

Theologians had written many treaties that tried to solve the breaking up of the theological concept of One God of Israel because of what had happened in Jesus Christ. Some of them were working on the line that was eventually accepted at Nicaea: God is One Trinity. This paradoxical close-up of One God has ever since been the fundamental teaching of Christian churches.

Trinity
The concept of Trinity is not really clear in the classic wording of the Nicene Creed:

We believe in one God, the Father Almighty, Maker of all things visible and invisible. 
And in one Lord Jesus Christ, the Son of God, 
begotten of the Father [the only-begotten; that is, of the essence of the Father, God of God], 
Light of Light, 
very God of very God, 
begotten, not made, 
being of one substance with the Father;
By whom all things were made [both in heaven and on earth];
Who for us men, and for our salvation, came down and was incarnate and was made man;
He suffered, and the third day he rose again, ascended into heaven;
From thence he shall come to judge the quick and the dead.
And in the Holy Ghost.

Faith in One God, Monotheism, the pillar of Judaism (Shema Israel) and absolute foundation of Islam.

They have it easy.

The section on Lord Jesus Christ is highly complex language and difficult to understand, at the limits of what human languages - even the delicate classical Greek - can express.


Arianism
In the Kingdom of Heaven there was a more sensible solution proposed by presbyter Arius (250–336) from the theological capital of Alexandria.

Faith in One God.

This must be the unshakeable foundation of true faith.

So why let this Jesus guy, admittedly a human born in the little town of Bethlehem, to break up this teaching of the One Majesty, God of Israel?

Arius relied on the holy Scriptures that underline that God is One and he noticed that Jesus himself says

"You heard me say, ‘I am going away and I am coming back to you.’ If you loved me, you would be glad that I am going to the Father, for the Father is greater than I."
John 14:28

Presbyter Arius began to look at the very beginning of everything and concluded that there must have been a time before Jesus Christ existed since he is called Son of God.

It is obvious, is it not, at least according to our human language and logic that if there is a Father and He has a Son, the Father was before the Son.


Wikipedia article summarizes all this nicely:

"Arius taught that God the Father and the Son did not exist together eternally.
He taught that the pre-incarnate Jesus was a divine being created by (and therefore inferior to) God the Father at some point, before which the Son did not exist.
In English-language works, it is sometimes said that Arians believe that Jesus is or was a "creature", in the sense of "created being". Arius and his followers appealed to Bible verses such as where Jesus says that the father is "greater than I" (John 14:28), and says "The Lord created me at the beginning of his work" (Proverbs 8:22) although this verse is now generally held by trinitarians to refer to some concept of "wisdom" rather than to the Son of God.
Of all the various disagreements within the Christian Church, the Arian controversy has held the greatest force and power of theological and political conflict, with the possible exception of the Protestant Reformation. The conflict between Arianism and Trinitarian beliefs was the first major doctrinal confrontation in the Church after the legalization of Christianity by the Roman Emperors Constantine I and Licinius."
Quoted from wikipedia


Tares among the wheat (darnel)
The teaching of Arius was not only a powerful catalyst in the development of Church teaching about Trinity that challenged the greatest minds in early Byzantine period from St Athanasius and St Augustine to the Cappadocian Fathers.

It was also tares among the wheat that spread among the wheat in the fields of Christ. The most significant believers who followed Arian teaching were the fearsome Goths approaching the borders of the Roman empire and eventually breaking in with devastating consequences to the classical world in events that changed the history of the world.

The growth of the Kingdom of Christ in Europe at these turbulent times was strongly influenced by Arian Christology from the regions of modern Yugoslavia all the way along the northern borders of Roman empire and to Spain.

A key person in this sowing of the darnel was Wulfila who brought many pagan Gothic tribes to Christ the Created One.


Arianism today
Because the teaching of Arius makes sense - of course Father is greater than Jesus, God is eternal, invisible, One - probably quite many baptised Christians today have similar ideas about the relationship between God and Christ at the back of their heads even when confessing the Nicene Creed by their mouths.

Trinity is a tough doctrine because it is so paradoxical and does not fit into the mighty power of our brains!

Go ask St Augustine if you do not agree with me on this!