Throughout the New Testament - which the Apostles and their disciples wrote under the direct leading of the Holy Spirit and which the assembly of believers all over the known world copied and studied - we have ALL the passages that were used by the attendees at Nicea to create that creed. We also have writings from leaders in the church going back to that first century arguing FROM Scripture proving the deity of Jesus. You're correct that heresies cropped up right away over the false doctrines rejecting that Jesus was God incarnate (3 guesses where they came from), but they were successfully defeated because they (and we) had the Scriptures and the illumination of them in the hearts of those believers to receive them. Why it is hard to explain is due to both the hardness of people's hearts and the efforts of Satan to cause confusion and corrupt the Gospel. Those who have surrendered their lives to Christ and who are led by the Holy Spirit accept the truths by faith.
What you say is true that Scripture was used to prove the deity of Jesus.
What developed was the understanding of how the God the of OT had become man. God was not human and could not be human as far as the Jews were concerned.
What also developed was the concept that there was one God in three persons.
Scripture does not clearly lay that out. It came from theology. Based in Scripture of course, but not found in Scripture in the terms and the way we know and understand it.