The word “Trinity” originated from Tertullian who was a pagan turned Catholic theologian around 200 AD. This word can have no relationship with Scripture as it came from the Platonic term “trias” which is philosophical in origin, and the Bible does not teach Greek philosophy. It is significant to note that Tertullian did not teach the Holy Spirit as a third being and he never taught the 3 in 1 god as the Trinity doctrine does today. So we know this doctrine developed in stages and could not have existed before 200 AD.
It was about 225 years “after” the death of the Apostles that the Nicene Council (325 AD) under the pressure of Emperor Constantine ruled in favour of Athanasius. The proponents were Arius who believed the Father and Son were two distinct beings, and Athanasius who argued a 3 in 1 god. Most Arians dared not challenge Constantine by refusing to sign the creed but Arius did and was exiled. However, Constantine later changed his mind in favour of Arius and allowed him to return. But Arius was killed on the way which Scholars suspect Athanasius arranged. In any case, it was now Athanasius who was exiled. The chosen view changed back and forth for decades with the governing view being based on who was in power at the time and their preference.
It was about 280 years “after” the death of the Apostles in 381 AD when the Holy Spirit was officially declared as a third being. The Trinity doctrine was now fully established as we know it today. The noted Trinitarian historian Adolph Harnack observed that up to 325 AD the majority of Christians insisted the Holy Spirit was not a third being. So neither the Apostles nor their associates who survived them, nor their descendants (the so called early Church Fathers) believed in the trinity.
So the trinity doctrine is a manmade doctrine from the Catholic Church which was formed in two parts. The first part was made up in 325 AD which states it is a 3 in 1 god. The second and final part was made up in 381 AD. This final part states that the Holy Spirit is a third being. Many people reject the 3 in 1 god part of this manmade doctrine but still accept the Holy Spirit part and yet both parts were made up by the Papacy. If you are going to be silly enough to believe in their manmade doctrine then you either accept both parts or reject both parts. How can you be half Trinitarian and half non-Trinitarian?
The battle over the different views went on for decades according to the politics of the Empire with more than a dozen councils held to try and solve the disputes. Whenever the Arians were dominant, they persecuted the Trinitarians, and when the Trinitarians were dominant, they persecuted the Arians. Blood literally flowed in the streets. By the 8th century, Arianism was almost eradicated. The Trinitarians proved to be more efficient in killing the Arians than the Arians in killing them. Had the Arians been more successful in killing, then the Trinity doctrine would probably not exist today. The final outcome was not the result of rational debate and pious scholarship, but power, politics and the shedding of blood.
So did the Apostles believe in the Trinity? How could they have. It is impossible for the Apostles to have believed or written about this manmade doctrine that did not exist for more than three lifetimes “after” their death. So if you think that Scripture shows a 3 in 1 god and the Holy Spirit to be another being, think again. It is more than likely a wrong idea you have been indoctrinated with as the Apostles could never have written about something that did not even exist in their time.