Why was Theodosius successful in ending the Arian Controversy?

While this is an interesting Q, which I UV, I find the available options severely limited.

• A Roman Emperor, supported by a faction in the church, or by
• A majority of church officials

This a is a secular view of events that somehow either assumes God’s inability or indifference to the church becoming disoriented after the Apostles departure.

We might liken the trinity gaining a prominent influence over the church the Apostles began under Jesus’ leadership, to the golden calf, the 12 spies, Cain, etc.

Clearly, God has designed in some interference and resistance to His plan for mankind. We only need to look to the serpent in the garden to see this reality. God allows confusion, temptation, distraction - the world is full of such things and we know whose world this is - for a time.

Thus, the reason why Theodosius was successful, apart from the sword or some contrived unity amongst 'church leaders’, is secondary to God allowing it to happen. Just as He used Babylon to bring severe trial on Israel for her persistent idolatry, He uses/allows evil to bring His true followers to the fore and in deeper relationship with Him.

Of those who oppose his [Trinitarian] edict, it said:

They will suffer … the punishment of our authority which … we shall decide to inflict.

Thirty-six Pneumatomachians arrived but were denied admission to the council when they refused to accept the Nicene creed

And yet, they did not seem to have suffered any (other) punishment.

• One cannot be accepted as part of a second (ecumenical) council without adhering to the first; this is not simply a question of (church) history, but of basic logic.
• The Pneumatomachians in question were not merely Pneumatomachians, but Arians also; i.e., they did not merely oppose the divinity of the Holy Ghost, but of the Son as well.

Theodosius expelled the Homoian bishop, Demophilus of Constantinople

Gregory of Nazianzus [...] presided over part of the Council and vehemently opposed any compromise with the Homoiousians

why did he have to exile the h*m*ian bishop of the capital of the empire (Constantinople) ?

Homoi(ousi)ans (implicitly) denied or opposed the first ecumenical council, which explicitly and categorically stated the Son to be h*m*-ousios, not (merely) h*m*i-(ousi)os, with His Father; however, the second council was convened to settle a different issue, namely whether the Holy Spirit, not the Son, was either h*m*-, or h*m*i-, or hetero-ousios in relation to the other two (whose sameness of substance has already been established previously, at Nicaea).

My (half-educated) guess would be as follows:

• either Theodosius hastily condemned that which Nicaea did not explicitly condemn as heretical, only to have been subsequently forced to convene a synod when he saw that many of those accepting Nicaea simply did not share his view or interpretation of it concerning the Spirit;
• or, alternately, the 380 edict was eventually edited or rewritten later (381+), to more faithfully or accurately reflect the new reality. (As you are probably aware, documents are constantly being copied, as older versions inevitably decay).

Why was Theodosius successful in ending the Arian Controversy?

• Part of it was timing. The West was already Trinitarian in 379, one year prior to his edict (Cunctos populos). Thus unity existed in the West!
• Part of it was thanks to Nicaea and the Nicene credal formulation.
• Part of it was that Edict of Thessalonica (Cunctos populos) which as jointly issued by Theodosius I, emperor of the East, Gratian, emperor of the West, and Gratian's junior co-ruler Valentinian II, on 27 February 380.
• Part of it were the various measures that the emperor imposed on heretics following the Edict of Thessalonica.

That made a great 4x4 recipe for destroying Arianism!

As is true in any conflict dealing with heresy, it never disappears over night and often takes time, sometimes centuries to disappear.

Here a little background is in order.

In February 313 AD, the Emperor Constantine I issued the Edict of Milan that allowed Christians To live benevolently within the Roman Empire. The Edict of Milan gave Christianity legal status and a reprieve from persecution but did not make it the state church of the Roman Empire. That occurred in AD 380 with the Edict of Thessalonica.

The strife within the Church did not end with Nicaea, and the Nicene credal formulation remained contentious even among anti-Arian churchmen. Constantine, while urging tolerance, began to think that he had come down on the wrong side, and that the Nicene with their fervid, reciprocal persecution of Arians were actually perpetuating the strife within the Church. Constantine was not even baptized until he was on his deathbed, choose a bishop somewhat sympathetic to Arius, Eusebius of Nicomedia, to perform the baptism. That speaks volumes as to the reigning climate of the times. The manner of Constantine baptism in both choice of administer and timeliness set an historical bad example in my opinion.

When Valens was succeeded by Theodosius I, in 379, Arianism was widespread in the eastern half of the Empire, while the west had remained steadfastly Nicene. Theodosius, who had been born in Hispania (Spain) remained a steadfast Nicene Christian. In August, his western counterpart Gratian promoted persecution of heretics in the west. The time for a decisive move was ready.

Edict of Thessalonica

The Edict of Thessalonica was jointly issued by Theodosius I, emperor of the East, Gratian, emperor of the West, and Gratian's junior co-ruler Valentinian II, on 27 February 380. The edict came after Theodosius had been baptized by the bishop Ascholius of Thessalonica upon suffering a severe illness in that city.

IMPPP. GR(ATI)ANUS, VAL(ENTINI)ANUS ET THE(O)D(OSIUS) AAA. EDICTUM AD POPULUM VRB(IS) CONSTANTINOP(OLITANAE).

Cunctos populos, quos clementiae nostrae regit temperamentum, in tali volumus religione versari, quam divinum Petrum apostolum tradidisse Romanis religio usque ad nunc ab ipso insinuata declarat quamque pontificem Damasum sequi claret et Petrum Aleksandriae episcopum virum apostolicae sanctitatis, hoc est, ut secundum apostolicam disciplinam evangelicamque doctrinam patris et filii et spiritus sancti unam deitatem sub pari maiestate et sub pia trinitate credamus. Hanc legem sequentes Christianorum catholicorum nomen iubemus amplecti, reliquos vero dementes vesanosque iudicantes haeretici dogmatis infamiam sustinere ‘nec conciliabula eorum ecclesiarum nomen accipere’, divina primum vindicta, post etiam motus nostri, quem ex caelesti arbitro sumpserimus, ultione plectendos.

DAT. III Kal. Mar. THESSAL(ONICAE) GR(ATI)ANO A. V ET THEOD(OSIO) A. I CONSS.

EMPERORS GRATIAN, VALENTINIAN AND THEODOSIUS AUGUSTI. EDICT TO THE PEOPLE OF CONSTANTINOPLE.

It is our desire that all the various nations which are subject to our Clemency and Moderation, should continue to profess that religion which was delivered to the Romans by the divine Apostle Peter, as it has been preserved by faithful tradition, and which is now professed by the Pontiff Damasus and by Peter, Bishop of Alexandria, a man of apostolic holiness. According to the apostolic teaching and the doctrine of the Gospel, let us believe in the one deity of the Father and of the Son and of the Holy Spirit, in equal majesty and in a holy Trinity. We order the followers of this law to embrace the name of Catholic Christians; but as for the others, since, in our judgment they are foolish madmen, we decree that they shall be branded with the ignominious name of heretics, and shall not presume to give to their conventicles the name of churches. They will suffer in the first place the chastis*m*nt of the divine condemnation and in the second the punishment of our authority which in accordance with the will of Heaven we shall decide to inflict.

GIVEN IN THESSALONICA ON THE THIRD DAY FROM THE CALENDS OF MARCH, DURING THE FIFTH CONSULATE OF GRATIAN AUGUSTUS AND FIRST OF THEODOSIUS AUGUSTUS

— Codex Theodosianus, xvi.1.2

Importance

The edict was followed in 381 by the First Council of Constantinople, which affirmed the Nicene Symbolum and gave final form to the Nicene-Constantinopolitan Creed.

The edict was issued under the influence of Ascholius, and thus of Pope Damasus I, who had appointed him. It re-affirmed a single expression of the Apostolic Faith as legitimate in the Roman Empire, "catholic" (that is, universal) and "orthodox" (that is, correct in teaching).

The Nicene Creed states: "We believe in one God, the Father Almighty ... And in one Lord Jesus Christ." It declares Jesus Christ be "consubstantial (h*m*-ousios) with the Father," which may be interpreted as numerical or as qualitative sameness (See Homoousion). The creed adds that we also believe in the Holy Spirit but does not say that the Holy Spirit is h*m*-ousios with the Father. The Edict of Thessalonica goes much further and declares "the Father, the Son and the Holy Spirit" to be "one deity ... in equal majesty and in a holy Trinity."

Enforcement of the Edict

After the edict in February 380, Theodosius spent a great deal of energy trying to suppress all non-Nicene forms of Christianity, especially Arianism, and in establishing Nicene orthodoxy throughout his realm:

"In January of the following year (381), another edict forbade the heretics to settle in the cities."

"In the same year, after the reformulation of the Nicene doctrine by the Council of Constantinople ... the procouncil of Asia was ordered to deliver all churches to these bishops 'who profess that the Father, Son and Holy Spirit are one majesty and virtue'."

In 383, the Emperor ordered the various non-Nicene sects (Arians, Anomoeans, Macedonians, and Novatians) to submit written creeds to him, which he prayerfully reviewed and then burned, save for that of the Novatians, who also supported Nicene Christianity. The other sects lost the right to meet, ordain priests, or spread their beliefs.

“The execution of Priscillian and his followers may be cited as typical of the treatment of heretics conditions in that time.” In 384, Priscillian was condemned by the synod of Bordeaux, found guilty of magic in a secular court, and put to death by the sword with a number of his followers.

Theodosius forbade heretics to reside within Constantinople, and in 392 and 394 confiscated their places of worship.

Ultimately, Theodosius did not wipe out Arianism completely along with all it’s controversy, but he definitely helped unify the East with West in the teachings of the Council of Nicaea of 325 with the Trinitarian dogma. The Arian controversy still lingers today.

For a start, Theodosius did not end the Arian controversy because it never did go away, and we have its modern-day counterparts still protesting against the way Arius and his supporters were dealt with by the early Church. They are still promoting variations on the theological theme of Arius.

I'm inclined to view your question as, "Why was Theodosius apparently successful in ending the Arian Controversy around the 380s?" His lack of lasting success is dealt with in this book, from which I quote relevant points giving an outline of the problems that kept growing, and which caused Theodosius to act so drastically. I suggest that it is impossible to understand the apparent success of Theodosius without grasping the protracted political and theological struggles from the time of Arius, and then to glance beyond the Council of Constantinople, 381:

"While Athanasius was in exile in Trier, Arius died - one day before he was to be restored as a Christian presbyter in a special ceremony in Constantinople in 336, only months before Constantine's own death on May 22, 337. Constantine lived as a pagan and died as an Arian...

Constantine's successor was his son Constantius... who ruled until his death in 362, [and] constantly hounded the bishop [Athanasius] who seemed the last key holdout for trinitarian orthodoxy against Arianism and semi-Arianism. The emperor wanted peace, and uniformity was its path, [wanting to] replace h*m*ousios in the Nicene Creed with h*m*iousios which means, 'of a similar substance' and would be acceptable to the semi-Arians...

"In all, Athanasius endured five exiles.' Seventeen years, out of forty-six as bishop, Athanasius had spent in exile. Politics and theology had ever intermingled...' 7... His own synod in Alexandria met in 362. The bishops gathered there reaffirmed h*m*ousios as the only proper description of the relationship of the Son of God to the Father and explicitly rejected both the semi-Arian h*m*iousios and Sabellianism as heresies... Athanasius died in 373 in Alexandria." The Story of Christian Theology, Roger E. Olson, pp 164-166, (Apollos 1999)

You mention the Council of Constantinople (381) as decisive, and certainly for all practical purposes the debate over the Trinity ended there (as far as the combined Catholic and Orthodox churches were concerned.) Yet just because the debate seemed to be finally sorted, that in no way caused the evangelism of Arianism and Sabellianism to grind to a halt, for those promoters remained active on the fringes of Christendom for a long time:

"Several Arian missionaries traveled to the so-called barbarian tribes of central Europe and evangelized them for Arian Christianity, and when some of those tribes contributed to the fall of the Roman Empire, Arianism reappeared in the centers of power - especially Rome and other places in the West...

As the Council of Constantinople adjourned and the bishops departed for their home sees, the differences and resentments between Alexandria and Antioch were just beginning to boil... The Council of Constantinople had made official the use of terms and concepts like nature and person for explaining the Trinity. Alexandrians... would argue more and more vehemently that just as the Trinity is one substance, or nature, and three persons so Jesus Christ is one nature and one person... When it comes to personhood, Antiochenes averred, two can become one while remaining two. The stage was set for a theological blowout... [which] came in 428." (Ibid. p 197 & 210)

So much for 'consensus'. Theodosius appeared to succeed in ending a particular debate, but he did not end the Arian controversy. His partial success required using theological and political clout, and bishops who had been battling for nearly 80 years were only too glad to support his decisive actions. They'd all had enough of the wrangling.

Finally, you have clearly done so much research yourself, it might be a waste of time anybody else progressing further along that tortuous route to try to give you additional, and helpful, information. However, I will urge you to look at the link below where at 3 review articles deal with Theodosius.

Article “Examination of the Rise of Homoean Theology and the Council of Constantinople in AD 360” 11 May 2013, by Steven Wedgeworth “My post entitled “The Myth of the Ecumenical Early Church” was actually taken from a paraphrase of a paper I wrote while in seminary in 2007. As a part of the assignment, my paper had to present a “problem” in church history…”

Article “Greenslade on the Dualism of Church and State, 14 June 2013 by E.J. Hutchinson “S.L. Greenslade, in his delightful little book Church and State from Constantine to Theodosius (Greenwood Press, 1981 [repr. of 1954 ed. by SCM Press]), which unfortunately suffers from the defect, significant especially given the book’s title, of providing readers a definition of neither “church” nor “state,” has some thought-provoking paragraphs about how the differences…”

Article, Defending Constantine (2) 29 Aug. 2013 by E.J. Hutchinson "Another note on Peter Leithart’s Defending Constantine. There seems to be some confusion at work in a couple of passages regarding Theodosius I and Theodosius II, by which I mean that actions undertaken by the latter seem to be attributed to the former (and possibly confusion over the Valentinians as well). Two instances…" https://calvinistinternational.com/page/2/?s=Theodosius