Was the Nicene Creed accepted under duress?

The main purpose of the Nicene Creed was to establish Trinitarianism as Christian doctrine, in opposition to Arianism. If a majority agreement was obtained under duress, few bishops would have been courageous enough to say so; after all, duress implies that you must remain silent about your disagreement with the verdict. Edward Gibbon says, in The Decline and Fall of the Roman Empire (D. M. Low abridgement, page 319), that when Emperor Constantine ratified the Nicene Creed, he made a firm declaration that those who resisted the judgement of the synod must prepare themselves for an immediate exile. That is implied evidence that Constantine was aware of the continued discontent. It was Eusebius of Nicomedia who wrote, "We have committed an impious act, O Prince, by subscribing to a blasphemy from fear of you." But his position was probably safer than most others.

Further evidence that the Nicene Creed might not have been accepted as unanimously as churches generally teach is that in spite of Constantine's decree, Arianism refused to go away. In 358 Constantine's son, Constantius, called two separate councils to debate the issue. The Council of Rimini, in Italy, and the Council of Seleucia, in the eastern empire, demonstrated that the church was still deeply split over the issue. A third council, at Constaninople, reaffirmed the Nicene Creed, but it was not until 381 that Emperor Theodosius convoked a general council in Constantinople, which no western bishop attended, that reaffirmed a somewhat reformulated Nicene Creed.

The Nicene Creed as formulated in the fourth century did not include the Filioque clause, which was a western innovation of later centuries.

Background

There are several key details missing from the existing answers. Despite offering a bounty on this question to see about getting some more complete answers, no additional answers were offered and existing answers were not revised. I therefore awarded the bounty to philipthegreat's well written answer. Because the additional details I was looking for were not added to the answers here, I have elected to add what I believe to be important missing details as a separate answer to this question myself.

Supplementary Details

While it is tempting to believe that Constantine influenced the decision of the council so that it resulted in a claim of Jesus Deity, despite only two dissenters (not including Arius) to the Nicene Creed, the emperor's cousin Eusebius of Nicomedia persuaded the emperor to exile the major proponents of the prevailing view of the council. One could argue that this indicates the Emperor's disfavor with the result of the council and his opposition to the Nicene Creed.

Furthermore, Emperor Valens, one of the emperors after Constantine was a noted Arianist and the persecution of Christianity was resumed under Emperor Julian until Emperor Theodosius I made Nicene Christianity the official state religion in the Edict of Thessalonica. All of these instances provided opportunities for the church to change to Arian beliefs or revive discussions should the Nicene Creed have been published disingenuously or under coercion.

Since no doctrinal reversals occurred, it seem unlikely that these doctrines would have persisted if they were arrived at disingenuously with such ample opportunities for reversal and for the "true" beliefs to come to light. Since no such reversals occurred, this leads to the conclusion that Nicene Creed was, in fact, the genuine result of the First Ecumenical Council.

The Nicene Creed is a long-standing tradition in Christianity, and "defines the mainstream definition of Christianity for most Christians". It has been independently accepted by the Catholic, Eastern Orthodox, Lutheran (pdf link), and plenty of Protestant churches.

Joshua Christian seems to have presented a fanciful (or at least misleading) view of the formation of the Nicene Creed. I could find no other sources that back up his story. Here is the account of the Creed's creation, as I understand it.

Sometime before 325 AD, around 1800 Christian bishops (every bishop Constantine knew existed) were invited to convene in Nicea. We have no way of knowing with 100% certainty how many bishops actually showed up, but the traditional number is 318. Athanasius of Alexandria counted 318; Eustathius of Antioch estimated 270.

Please note, this is only a count of bishops. Each bishop was allowed (and encouraged) to bring deacons, priests, or other traveling companions. Only the bishops had voting rights. Therefore, it is entirely possible that there were 2030 total people in attendance, with only 318 votes cast. Only 318 votes could be cast, because only that number of people had voting authority.

Addressing Constantine's supposed power over the council: I will concede the point that Constantine wanted a Creed to be developed in order to "forge a 'unified' church". That seems likely, by the fact that he arranged for the council to meet. However, I disagree that Constantine cared that this particular Creed pass. At this time, Constantine was not even Christian (he was baptized 12 years later, in 337 AD)! It would make sense for him to go along with whatever Creed naturally came about during this council.

As further proof that Constantine's meddling was not the theological backing of the Nicene Creed, there was a Second Ecumenical Council that met in 381 AD (40+ years after Constantine died) and confirmed the Nicene Creed. It should be noted that the Second Ecumenical Council did not have overlapping members with the First Ecumenical Council, as it was held 56 years later.

Addressing the Arians. Arianism was, indeed, a major topic for the Council of Nicea. The debate centered on whether Jesus co-existed eternally with God the Father and the Holy Spirit (a trinitarian view) or whether Jesus was created by God the Father at some point prior to the creation of the universe (a non-trinitarian view). Arius, a priest in the Alexandrian diocese, claimed that Jesus was "born" and was, therefore, different in essence from God the Father.

Initially, 22 of the 318 bishops came out in support of Arianism. However, after the writings of Arius were read in the council, and after hearing arguments from many of the other bishops, 19 of the 22 Arians came to believe in a trinitarian view of the Godhead. Arguments that swayed toward the trinitarian view included John 10:30, where Jesus says "I and the Father are one" and John 1:1, which says "In the beginning was the Word, and the Word was with God, and the Word was God." Each side of the debate had ample time to make a case: the debate lasted for over two months!

In the end, the majority of the 318 bishops agreed upon the trinitarian view presented in the Nicene Creed. Arius was exiled, along with Theonas and Secundus, to Illyricum. None of those three were bishops, so they did not have voting authority in the council.

Three bishops, Theognis of Nicaea, Eusebius of Nicomedia, and Maris of Chalcedon, remained believing the Arian view of the Godhead. They chose to sign the Nicene Creed anyway, out of deference to Constantine.

In the end, all 318 bishops in attendance signed the Nicene Creed. Out of those, only 3 bishops chose to sign without fully believing the Creed themselves. There were only three people exiled (and those, only to another province in Rome).

Joshua Christian misses the forest for the trees. He is correct that there was political and religious pressure. He far overstates that effect on the Creed as a whole.