"Unitarian" could refer to anyone who believe that only 1 God exists. This article mainly focus on Strict Unitarians who believe:

1. There is only 1 God, the Father.
2. Jesus Christ, who:
   • Is not God, nor a mode, nor a part
   • Only existed since his birth (no incarnation or pre-existence)
   • Is the Son of God since his birth and because his Father is God Himself
   • Is a perfect and sinless human
   • Is worthy to be worshiped (respected) as a king and as the Christ, but not as a god
   • Should not receive prayers as a god
   • Has risen and is alive and has returned to God (not to be confused with the Holy Spirit of God that is still present)
3. The Holy Spirit is not a person, but the Spirit part of God which is experienced as God's presence or power.

History

From the historians' perspective, the Unitarian movement is relatively young:

Scattered expressions of anti-Trinitarian opinion appeared in Poland early. At the age of 80, Catherine, wife of Melchior Vogel or Weygel, was burned at Cracow (1539) for apostasy; whether her views embraced more than deism is not clear. The first synod of the (Calvinist) Reformed Church took place in 1555; the second Synod (1556) faced the theological challenges of Grzegorz Paweł z Brzezin (Gregory Pauli) and Peter Gonesius (Piotr z Goniądza), who were aware of the works of Servetus and of Italian antitrinitarians such as Matteo Gribaldi. The arrival of Giorgio Biandrata in 1558 furnished the party with a temporary leader. -- Wikipedia

However, Unitarians belief that humans have been Unitarians since Adam who walked with only 1 God (Genesis 3:8) and that it has been officially documented since Moses as mentioned above.

Biblical Unitarians

These group of Unitarians strictly base their beliefs on their interpretation of the Christian Bible.

Critique

Trinitarians which is the majority of Christians criticizes the Unitarian view on the following points:

Unitarianism changes God's character

Chris Date argues that if the Unitarian view is true, then the Father would be guilty of:

1. Child abuse, by sending Jesus to be slaughtered for our sins.
2. There would have been no humility or sacrifice to God if someone else is sacrificed.

For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life. -- John 3:16 (NKJV)

Dr. Dale Tuggy defended this argument that

1. Jesus was an adult at the time of the crucifixion and had a free will and option to escape the crucifixion, yet choose to complete his task to glorify his Father.
2. It is a contradiction if a perfect holy infinite sinless Almighty God, that cannot be tempted, take sin upon Himself and die a human death. That would disqualify Him from these qualities.

And he (Jesus) was withdrawn from them about a stone’s throw, and he knelt down and prayed, saying, “Father, if it is Your will, take this cup away from me; nevertheless not my will, but Yours, be done.” Then an angel appeared to Him from heaven, strengthening Him. -- Luke 22:41-43 (NKJV)

Although Jesus knew what suffering was awaiting him, he still did not think that his Father was cruel or unfair:

Jesus spoke these words, lifted up his eyes to heaven, and said... “Father, I desire that they also whom You gave me may be with me where I am, that they may behold my glory which You have given me; for You loved me before the foundation of the world. O righteous Father! The world has not known You, but I have known You; and these have known that You sent me. And I have declared to them Your name, and will declare it, that the love with which You loved me may be in them, and I in them.” John 17:24-26 (NKJV)

How can God be irresponsible to give ultimate authority to one human?

God did not retire or handover all responsibility or power to Jesus. Instead, Jesus serve God.

Far fewer Unitarians than Trinitarians in the world

There are only about 14,750,000 Unitarians in the world compared to about 2,604,381,000 Christians. That is only about 1 out of every 177 Christians or 0.6% of the Christian population.

Universal Unitarians

They do not care what you believe about Jesus or God, everyone is welcome as long as you support their 7 principles:

As Rev. Barbara Wells ten Hove explains, “The Principles are not dogma or doctrine, but rather a guide for those of us who choose to join and participate in Unitarian Universalist religious communities.”

1. The inherent worth and dignity of every person;
2. Justice, equity and compassion in human relations;
3. Acceptance of one another and encouragement to spiritual growth in our congregations;
4. A free and responsible search for truth and meaning;
5. The right of conscience and the use of the democratic process within our congregations and in society at large;
6. The goal of world community with peace, liberty, and justice for all;
7. Respect for the interdependent web of all existence of which we are a part.

-- Unitarian Universalist Association

The beliefs of individual Unitarian Universalists range widely; they can include, but are not limited to, Religious humanism, Judaism, Christianity, Islam, Hinduism, Sikhism, Buddhism, Taoism, Syncretism, Neopaganism, Atheism, Agnosticism, New Age, Omnism, Pantheism, Panentheism, Pandeism, Deism, and teachings of the Baháʼí Faith. -- Wikipedia