If the early Christian church had believed Jesus to be simply human, they would not have developed a doctrine of the Trinity. If we ask, "Why did the Christians come to understand God so differently?", the answer revolves around Jesus. That is to say: the conversation in the early church started with Jesus and how they understood Jesus.
There was a separate conversation about the Holy Spirit, and that conversation took place slightly later in church history. By the time that conversation came into focus for the Christian community, there had already been centuries of conversation about how to understand Jesus. There seems to have been an expectation by that point in time: whatever had been decided about Jesus, the same logic should apply to thinking about the Holy Spirit. They brought parallel reasoning to the question of how to understand the Holy Spirit, and eventually reached the doctrine of the Trinity as it is known today. As with any summary, that is simplified; the relevant point is that the Christian experience of the Holy Spirit did not demand the same conversation. The church's experience of the Holy Spirit did not demand any changes to the idea of One God.
To make my point clearer, think for a moment about the Holy Spirit. For those who take guidance from the Bible, the Spirit is clearly non-human, clearly uncreated, clearly based solely in God. In church history I have not seen any serious Christian disputes about the eternal divine nature of the Holy Spirit. So why didn't the church's experience with the Holy Spirit drive the conversation about the Trinity? And consider that for centuries Judaism had lived comfortably with awareness of the Holy Spirit without questioning the oneness of God.
The answer seems to lie here: Many viewed the Holy Spirit as something like an extension of God, and not as fully distinct from God. While that question does not arise with Jesus, it is an important question in our understanding of the Holy Spirit.