The council of nicea in Rome decided what Christians could believe
Actually it was mainly about Arianism. More importantly, it decided a tiny handful of issues, not "what Christians could believe". It didn't determine the canon (the "official" list of books included in the bible) or even settle the issue of Arianism.
How can we twist Christs words to benefit the Roman state
The "words of Christ" were the problem and the were not twisted for any Roman benefit. The central issue began as early as John (perhaps earlier) and continued long after Nicaea. It concerned Jesus' nature. Arius asserted that God the father existed before Jesus and that Jesus' was of a lesser, different "substance" than God the father. However, in John we find "I and the father are one". But elsewhere we find statements which indicate that Jesus is not co-equal with the father. Nicaea didn't even resolve this Christological issue, let alone have any impact on the words in Christian texts (or which texts would be included in the bible and which wouldn't).
the rejection of esoteric forms of Christianity
...were never part of Nicaea or really a concern of any councils. Rather, those like Marcion, Valentinus, the Sethians, etc., were countered through publications such as Irenaeus'
Adversus haereses.
The term "gnostic" is largely a modern one and mostly a mistake. First,
gnostikos ("gnostic"), although it existed in antiquity and was used to describe some people, beliefs, and/or groups now usually called "gnostics", the people who applied the term were heresiologists like Irenaeus, not the people now usually called "gnostics". In other words, to the extent "gnostics" (
gnostikoi) existed as a category in early Christianity, we have no evidence that they used this label to describe themselves. Second, the label is used inconsistently even by heresiologists. Third, most groups, persons, beliefs, etc., categorized today as "gnostic" were never called "gnostics" by anyone. Fourth, there is disagreement over which groups/persons/beliefs were gnostic and which weren't (including some who argue the term is anachronistic and not only inaccurate but very misleading).
Most heresies in the first centuries of Christianity were dealt with by biships and other Church leaders, through texts arguing against specific "heresies", and through deliberate schisms "esoteric" Christians imposed themselves. The councils, from Nicaea all the way to Trent and beyond, never dealt with "esoteric Christianities" as
1) Many "gnostics" weren't Christian
2) Those that were frequently separated themselves from more mainstream Christianity and Ecclesiastical structures
3) Those that were addressed by the Church through "official" channels didn't concern Rome
4) The Emperors couldn't care less about pockets of groups like the Sethians, Carpocratians, Valentinians, etc., because they posed no threat to the empire. One of the side-effects of claiming secret, special knowledge or promoting radical lifestyles is that you will tend either not to attract many followers, deliberately limit followers, or both. Arius wasn't proposing esoteric notions, but threatening Christianity as monotheism while promoting ideas about the nature of both God and Jesus that were
too close to mainstream conceptions for comfort. The first councils concerned the nature of Jesus in relation to god the father and then in relation to the holy spirit.
like Romans often did with many previous religions.
They really didn't. In fact, there is probably no time in or place in history in which a greater number of diverse religious practices existed in one "place" (i.e., the Roman empire).