Q - How did the Testimony of Prophet Muhammad (PBUH) prove that Arius was right and the real follower of Jesus Christ?

A - First Council of Nicaea:

In the early centuries of Christianity there was nothing called “Jesus as Son of God” before the Roman Empire being converted to Christianity, but unfortunately they forced the priests to alter their believes from saying “Messenger of God” to be “Son of God”, similar to Romans ancient pagan religion.

The story is mentioned in the article of “First Council of Nicaea” from Wikipedia, where it mentions that following:

 [The council did not create the doctrine of the deity of Christ as is sometimes claimed but it did settle to some degree the debate within the early Christian communities regarding the divinity of Christ. This idea of the divinity of Christ along with the idea of Christ as a “messenger from the one God” (The Father) had long existed in various parts of the Roman empire. The divinity of Christ had also been widely endorsed by the Christian community in the otherwise pagan city of Rome.

One purpose of the council was to resolve disagreements arising from within the Church of Alexandria over the nature of Jesus in relationship to God the Father; in particular, whether Jesus was the literal son of God or was he a figurative son, like the other "sons of God" in the Bible. St. Alexander of Alexandria and Athanasius claimed to take the first position; the popular presbyter Arius, from whom the term Arianism comes, is said to have taken the second.

The council decided against the Arians overwhelmingly (of the estimated 250–318 attendees, all but two voted against Arius.] Wikipedia

B – But, who is Arius?

Also, from Wikipedia (“Arius”) we know that:

[Arius (Ἄρειος, AD 250 or 256 – 336) was a Christian presbyter from Alexandria, Egypt.

His teachings about the nature of the Godhead, which emphasized the Father's Divinity over the Son, and his opposition to the Athanasian or Trinitarian Christology, made him a controversial figure in the First Council of Nicea, convened by Roman Emperor Constantine in 325 AD.

 After Emperor Constantine legalized and formalized the Christianity of the time in the Roman Empire, the newly recognized Catholic Church sought to unify and clarify its theology.

Trinitarian partisans, including Athanasius, used Arius and Arianism as epithets to describe those who disagreed with their doctrine of co-equal Trinitarianism, a Christology representing the Father and Son (Jesus of Nazareth) as "of one essence" (consubstantial) and coeternal.

………………

[The discussions of First Council of Nicaea]

For about two months, the two sides argued and debated, with each appealing to Scripture to justify their respective positions.

Arius maintained that the Son of God was a Creature, made from nothing;

………………..

and thus "were He in the truest sense a son, He must have come after the Father, therefore the time obviously was when He was not, and hence He was a finite being."

Arius appealed to Scripture, quoting verses such as John 14:28: "the Father is greater than I". And also Colossians 1:15: "Firstborn of all creation."

Thus, Arius insisted that the Son was under God the Father, and not co-equal or co-eternal with Him.

According to some accounts in the hagiography of Nicholas of Myra, debate at the council became so heated that at one point, Arius was slapped in the face by the saint (the same St. Nick called "Santa Claus").

Under Constantine's influence, the majority of the bishops ultimately agreed upon a creed, known thereafter as the Nicene creed. It included the word homoousios, meaning "consubstantial", or "one in essence", which was incompatible with Arius' beliefs.

On June 19, 325, council and emperor issued a circular to the churches in and around Alexandria: Arius and two of his unyielding partisans (Theonas and Secundus) were deposed and exiled to Illyricum, while three other supporters—Theognis of Nicaea, Eusebius of Nicomedia and Maris of Chalcedon—affixed their signatures solely out of deference to the emperor. ] Wikipedia

C - Prophet Muhammad (PBUH) mentions “Arius” in his message to Heraclius:

The following is the message sent from Prophet Muhammad (PBUH) to Heraclius the Roman Empire in Syria:

بسم الله الرحمن الرحيم

من محمد عبدالله ورسوله، إلى هرقل عظيم الروم . سلام على من اتبع الهدى .

أما بعد: فإني أدعوك بدعاية الإسلام: أسلم تسلم يؤتك الله أجرك مرتين . فإن توليت فعليك إثم الأريسيين

و "يَا أَهْلَ الْكِتَابِ تَعَالَوْا إِلَى كَلِمَةٍ سَوَاءٍ بَيْنَنَا وَبَيْنَكُمْ أَلا نَعْبُدَ إِلا اللَّهَ وَلا نُشْرِكَ بِهِ شَيْئًا وَلا يَتَّخِذَ بَعْضُنَا بَعْضا أَرْبَابا مِنْ دُونِ اللَّهِ فَإِنْ تَوَلَّوْا فَقُولُوا اشْهَدُوا بِأَنَّا مُسْلِمُونَ"

English Translation:

In the name of Allah, the Compassionate, the most Merciful. From Muhammad Allah’s slave and his Messenger

To Heraclius*, the Roman Emperor, Peace be upon him who follows the guidance. Then,

I call you to Islam message. Accept Islam you’ll be safe, and ALLAH will give you your reward twice, but If you forsake [the message of God] you will carry the sin of all Arians (Arisiyn).

(Say: "O People of the Book! Come to common terms between us and you: that we worship none but Allah; that we associate no partners with Him; that we erect not, from among ourselves, Lords and patrons other than Allah. If then they turn back, say you: 'Bear witness that we are Muslims (submitting to Allah's Will.) (3:64)

The real message of Prophet Muhammad as it’s kept till today

----------------------------------------------------------------------------------------

* He was the Emperor of the Byzantine Empire [610-641] who captured Syria, Palestine, and Egypt from Persia [613-628].

Mentioning the word of “Arisiyn” which means “The Arians” in the sentence of: “but If you forsake you will carry the sin of all “Arians” was always difficult to interrupt by Muslims scholars, and most of them thought that means: The followers or citizens of the Roman Empire, and some interrupted it as the local name of Syrian farmers !!

But actually, even the messenger himself who carried the message didn’t know what the meaning of this word “Arisiyn”.

It means the “Arians”, followers of “Arius” who was exiled, fought, his books were burned, and all his followers were attacked for his beliefs, but mentioning his name in prophet message after almost 300 years of his death, is a miracle by prophet Muhammad, where even for those who lived in Christianity it was difficult to them to recognize such name, as it was forbidden to mention it, knowing that all his followers were totally extinct at least a century before Prophet Muhammad’s days.

So, how did the prophet knew this name and from where?

There is no answer except that it was revealed by God to him, and by adding the sentence that “but If you forsake you will carry the sin of all Arians”, it clearly means that if you kept forcing the Arians or real Christians to change their beliefs to the new one that includes the divinity of Christ and the trinity, then you will carry all of their sins on the day of judgment, as ALLAH says in Quran:

[They may bear their own burdens in full on the Day of Resurrection, and also of the burdens of those whom they misled without knowledge. Evil indeed is that which they shall bear!] (An-Nahel - 25)

And by this message, Prophet Muhammad (PBUH) clearly stated from God that Arius beliefs were correct and this man was truly defending the religion of Jesus (PBUH).

The following video is first part of Ar-Ressala movie [Story of Islam - The message 1/21 on youtube], where in its first scene it shows how 3 messengers are delivering the messages of ALLAH’s Apostle Muhammad (PBUH) to the Roman Empire, Khosrau of Persia and the king of Egypt.