Do all Christian denominations refer to Christ as God? Aren't there some that think he was just a man?

No, the doctrine of the Trinity and the Incarnation is orthodox Christian belief that has endured from the earliest days of Christianity. Throughout history, anyone who has denied these teachings has been considered heretical and outside of Christianity, not as just another denomination of Christianity. The orthodox understandings of the Trinity and the Incarnation, as described in the great creeds of the early church, are an integral part of what make us distinctively Christian, they always have been.  

But where are those doctrines in the Bible?

The teaching of the Trinity and the Incarnation is clear in the Scriptures, and they do not need to be added to, but because people started to stray from a Scriptural understanding of the nature of God there was a need to clarify and formalise what was true. In 1 Timothy 4:1-4, Paul warns Timothy that this will happen and instructs him to “2 preach the word; be prepared in season and out of season; correct, rebuke and encourage – with great patience and careful instruction. 3 For the time will come when people will not put up with sound doctrine. Instead, to suit their own desires, they will gather round them a great number of teachers to say what their itching ears want to hear. 4 They will turn their ears away from the truth and turn aside to myths.” The creeds then were statements made to carefully instruct the truth of Scripture because people were turning away from truth to falsehood. In particular, the Athanasian Creed and the Nicene Creed were written in response to a heresy which came from people like Arius, who was teaching that God the Son was a created being, and not equal to God the Father.

It is impossible in this answer to cover all the Scriptures which cover these topics, but let’s just take two verses as an example, which shed light on both the doctrine of the Trinity and the Incarnation.

Deuteronomy 6:4 “4 Hear, O Israel: the Lord our God, the Lord is one.”

This Deuteronomy verse is the fundamental verse of Jewish Monotheism called the Shema. All Jews knew this verse off by heart, for them it was the defining statement of orthodox belief in the God of the Scriptures, the maker of heaven and earth, the God of Abraham, Isaac and Jacob. The Jews knew that there were no other Gods, that the gods that the other nations worshipped were either man made inventions or demonic manifestations. (See Paul’s discussion in 1 Corinthians 8 and 10) There is one God whose name is the Lord – YHWH - and he alone has existed for eternity, as His name means ‘I Am who I Am’ (Exodus 3:14-15).

Paul and the other apostles were Jewish monotheists. They absolutely held to the truth of one God who was uncreated, it would have been completely unacceptable to them to believe in two or three ‘gods’ (even if they were considered  ‘lesser’ gods) as it would deny the Old Testament teachings that there is one God – the Lord.

However, we see in the New Testament three persons who are all know as God, and in particular we see Jesus Christ known as and repeatedly called ‘the Lord’, and uses the term ‘I Am’ to refer to himself as the God of Exodus 3:14. In 1 Corinthians 8:6, the apostle Paul deliberately re-works the Shema to show that Christ is ‘the Lord’.

1 Corinthians 8:6 “6 yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live.”

It forms like this:

The Lord our God            The Lord is One

One God, the Father       One Lord, Jesus Christ

Paul is quite clear that both the Father and the Son are the Lord God of the Old Testament. This was something Christ himself taught, and his disciples repeated. Christ regularly used the ‘I Am’ saying of himself, which left his Jewish monotheistic opponents in no doubt as to his claim of equality with God (See e.g. John 8:58, John 10:33-38, John 5:18). It was ultimately this charge of blasphemy that led to his death on the cross.

On top of this, his disciples affirmed that he was indeed both the Lord and God of the Old Testament. The verses below are some examples of this:

Acts 10:36 “You know the message God sent to the people of Israel, announcing the good news of peace through Jesus Christ, who is Lord of all.”

John 20:27-29 “27 Then he said to Thomas, ‘Put your finger here; see my hands. Reach out your hand and put it into my side. Stop doubting and believe.’ 28 Thomas said to him, ‘My Lord and my God!’ 29 Then Jesus told him, ‘Because you have seen me, you have believed; blessed are those who have not seen and yet have believed.’

Titus 2:13-14 “13 while we wait for the blessed hope – the appearing of the glory of our great God and Saviour, Jesus Christ, 14 who gave himself for us to redeem us from all wickedness and to purify for himself a people that are his very own, eager to do what is good.”

Jude 1:4 “For certain individuals whose condemnation was written about long ago have secretly slipped in among you. They are ungodly people, who pervert the grace of our God into a licence for immorality and deny Jesus Christ our only Sovereign and Lord.”

The Scriptures are quite clear. There is one God and he is three persons – eternally co-existing as a Trinity. Monotheism is clarified as Trinitarianism in the New Testament, and Jesus Christ is affirmed as the God of the Shema, the Sovereign Lord God of Israel. The creeds merely affirm what the Bible has as it’s clear position.