The Trinity as a biblical doctrine has a long and rich history. It was not formalized as a theological doctrine immediately in the early church, but that is not to say it was not held by the earliest Christians to be the truth.
The earliest Christians (and I believe this is to their credit) were not as concerned with systematic theology and intellectual doctrines as they were the practical Christian life. For this reason it took several hundred years before someone took the time to write out a formal explanation of the doctrine of the Trinity. This came to us via the Latin church leader Tertullian (not, as many heretics allege, from the Roman Catholic church).
For the committed and disciplined student of scripture, there can be no real question about it: the Trinity is the clear teaching of the Bible when all is taken together in context. However, since the Bible is not written as a textbook on systematic theology, we find these various statements scattered throughout in different places. False teachers have no trouble seizing on individual passages, taken out of context, to try to cast doubt on the Trinity. There are many sects out there which deny the Trinity, including Jehovah’s Witnesses, 7th Day Adventists, Mormons, some Messianic Jewish groups, Unitarians, Oneness Pentecostals, Mohammedanists/Muslims, and Arianists.
I have created this set of scripture references in order to make clear the fact that there is one God who exists in three Persons- the Trinity.