..depends which group of Christians you refer to..
That's true.
Adoptionism – believes Jesus became God, being born human, but adopted and deified by God the Father at His baptism.
Arianism – believes that Jesus is God, but unlike the Unbegotten or Uncreated Father, 'there was a time when he was not', and that the Son was begotten of the Father.
"but that by his own will and counsel he has subsisted before time and before ages as perfect as God, only begotten and unchangeable, and that before he was begotten, or created, or purposed, or established, he was not. For he was not unbegotten. We are persecuted because we say that the Son has a beginning but that God is without beginning." ( Theodoret: Arius's Letter to Eusebius of Nicomedia).
Docetism – believes Jesus was God, but not human, that his was a spiritual substance and only appeared to be flesh and blood.
Marcionism – believes similar to Docetism.
Ebionites – believe, like adoptionists, that Jesus was human but adopted by God at His baptism.
Gnosticism – that's a whole other ballgame. Different schools have different ideas, but generally Jesus is a Divine Being of Light, sent into the world to lead humanity from darkness.
Montanism – arose in the late 2nd century as a belief in the New Prophecies delivered by Montanus and his two female colleagues, Prisca and Maximilla, claiming inspiration by the Holy Spirit. Their followers claimed they were part of a prophetic succession originating with the daughters of Philip the Evangelist.
Trintarianism was eventually enforced by Roman rulers, declaring any other creed as heretical and punishable.
Old ground, but just for balance, a brief look at the allegiance of the Roman emperors:
Constantine the Great 306-337 – Nicene, but baptised by an Arian bishop.
(Now we have eras of co-emperors, east and west, and immediately now, three co-rulers
337-340CE Constantine II – Nicene
337-350CE Constans – Nicene – although thought largely to aggravate Constantius.
337-361CE Constantius II – Arian
Constantius exiled Nicene bishops, and when sole emperor (355CE) extended his pro-Arian policy to the western provinces with force, exiling Pope Liberius and installing Antipope Felix II.
At the Arian Third Council of Sirmium in 357CE led to dissent within the ranks and three camps evolved to oppose the Nicene Creed.
The first rejected Arius and his teaching and accepted the equality and co-eternality of the persons of the Trinity (Semi-Arian).
The second avoided invoking the name of Arius, but in large part followed Arius's teachings and described the Son as being like the Father.
The third group explicitly invoked Arius and described the Son as unlike the Father.
(Constantius wavered in his support between the first and the second party, while harshly persecuting the third.)
The debates among these groups resulted in numerous synods, in 343, 358 and two in 359, with no fewer than 14 further creeds issued between 340 and 360, leading a pagan commentator to declare "The highways were covered with galloping bishops."
Saint Jerome famous wrote of this time that the world "awoke with a groan to find itself Arian."
361-363CE Julian – pagan in favour of Hellenic Neoplatonism (aka Julian the Apostate
/ Julian the Philosopher)
Julian declared no favour either way and allowed all exiled bishops to return, increasing dissent all round.
363-364CE – Jovian – generally pro-Christian without bias.
364-375CE – Valentinian I – again even-handed with regard to Christianity.
364-378CE Valens – Arian – revived Constantius's persecution of Nicene believers.
379-395CE Theodosius – orthodox.
381CE Council of Constantinople declares a Triune creed.
Theodisius emerged as sole emperor and pro Nicene, although he appointed pagans to high offices of state.
So after Constantine what we see is a brief era of pro-Nicene orthodoxy in half the empire, then either Arian or not bothered, until Theodosius.
What is clear is that the emperors tried but could not settle the Nicene/Arian dispute in their favour (peace at any price) and that Arianism failed in the empire because it broke into factions and wore itself out.
Nicene orthodoxy won because it remained constant across the period in question.
Meanwhile, across borders, Arianism's flowering under Constantius saw Gothic convert and Arian bishop Ulfilas on a mission to the Gothic tribes across the Danube. The Goths were converted to Arianism, which spread among other Germanic tribes – Vandals, Langobards, Svevi, and Burgundians). When these tribes invaded the Western Roman Empire they founded their own Arian kingdoms.
In these kingdoms arose separate Arian and Nicene Churches, with parallel hierarchies, each serving different sets of believers. The Germanic elites were Arians, the majority population was Nicene. There was a general policy of toleration between Christians that included the Jews.
Towards the end of the 5th century, Arianism was giving way to Trinitarian orthodoxy, and ceased to have any meaningful presence by the 8th.