Arianism is a doctrine that came from Arius, a priest who taught in Alexandria. To many Christians, the teachings of Arianism are heretical and are not the correct Christian teachings as they deny that Jesus was of the same substance of the God of this monotheistic religion, making it one of the more prominent reasons Arianism has stopped being practiced today. Many may say and have claimed Arianism a powerful part of Christianity from the 4th to the 7th centuries AD however being against fundamental Christian Beliefs, such as that previously mentioned, this rhetoric was opposed by multiple theologians of the era as well as the early church(later Catholic Church).  

Arianism simply teaches that Jesus was not God. For this reason, Arianism opposes the dogma of the Holy Trinity. In 325 AD, the Nicene Creed was made, which the early Christians used to defeat Arianism, with the statement: "We believe in one Lord Jesus Christ, the Son of God...begotten, not made, consubstantial with the Father."

Belief 

The main difference between the beliefs of Arianism and other main Christian denominations is that the Arians did not believe in the Holy Trinity, which is a way that other Christian churches use to explain God. The information that we have today comes from the writing of people who were against Arianism and said the beliefs were wrong. These writings say that Arianism believed:

 Only God the Father is truly God. He alone is not-born, and is eternal. He does not change.
 God did not create the world by himself. He had help from something called Logos (The word). This Logos was created to create the world. This is God's "communication" with the world.
 God's communication with the world is known as the "Son of God" or the "Christ". The Son of God existed before he helped his Father make man.  He is the perfect image of God the Father. There is one exception though: Unlike God the Father, he was created (from nothing) by the will of the Father. This happened at the beginning of time. Therefore, he is not eternal. There was a time when he was not there. In the same way, his power, his wisdom and his knowledge are limited.

Between the First Council of Nicaea in 325, and the First Council of Constantinople in the year 381 there were no less than 18 different Creeds used by Arianists.

Antitrinitarianism today 
In the 16th century there was again a movement against the belief in the Holy Trinity.  Fausto Sozzini (1539-1604) was well known for teaching this as well as Ferenc Dávid (1510s - 1579). These movements developed into what is called Unitarianism today. Unitarians believe that there is only one person in god, not the three-in-one dogma of trinity. Some of the teachings of Jehovah's Witnesses are close to some of the things Arians believed. It is different only in that Arians believed the holy spirit is a person and Jehovah's Witnesses teach it is not a person, but a force God uses to do His will. It would however be false to call them Arians, as they reject many Arian teachings. Mormons and Oneness Pentecostalism also reject the Holy Trinity but for different reasons other than Arianism.

Related pages 
 Unitarianism - the (modern) belief that there is no Holy Trinity.

Nontrinitarianism
Ancient Christianity