The most honest answer is that nobody knows. The paucity of data about the earliest Christian movement makes all explanations necessarily reliant on conjecture and there are many competing hypotheses ranging all the way from maximalist conservative fundamentalism (Jesus really was God just like the Bible says) to full on mythicism (Jesus was originally only perceived as a celestial figure like an angel, later "historicized" as a real person).

Most scholarship is between those two extremes. A basic outline of the general mainstream view (which is kind of loose. Scholarship is very contentious on just about every aspect of Christian origins) is that Jesus started as a healer and exorcist, probably after being baptized by John, may or may not have claimed to be the Messiah acquired a following and was crucified by the Romans in Jerusalem, probably in response for some kind of perceived assault on the Temple during Passover.

At some point after that his followers came to believe he had been "raised" by God to heaven and that he would soon descend from the clouds as the "son of man" figure referenced in the Book of Daniel.

There are a diversity of arguments as to when Jesus became perceived as identical to God. The Gospels seem to show evidence of a trajectory of Jesus first being believed to have been adopted and exalted by God after his crucifixion (speeches given to Paul and Peter in Acts) , then at his baptism (in Mark), then being born already as the son of God (Matthew and Luke), then as the preexistent Logos (John). So as years go by, Jesus keeps getting more and more divinized until he's God himself. However, there is a verse in Philippians 2 which predates all the Gospels and shows what scholars call an "early high Christology").

Have this mind among yourselves, which is yours in Christ Jesus,1 6 pwho, though he was in qthe form of God, did not count equality with God a thing to be grasped, 7 but emptied himself, by taking the form of a servant,2 being born in the likeness of men. 8 And being found in human form, he humbled himself by vbecoming obedient to the point of death, even death on a cross. 9 Therefore God has highly exalted him and bestowed on him the name that is above every name, 10 so that at the name of Jesus bevery knee should bow, in heaven and on earth and under the earth, 11 and devery tongue confess that Jesus Christ is eLord, to the glory of God the Father.

This is interpreted by many as indicating that Paul thought Jesus was God from the beginning, but other scholars dispute this. They mostly do agree that the language is not Paul's, but disagree on whether this means Paul was quoting an earlier creed or whether it was interpolated by a later writer. There are heavyweight scholars who have argued both ways.

Looking at the language very carefully (especially in the Greek), the passage at least can be read as Paul seeing Jesus as preexistent, but still as distinct from God - "he did not find equality with God a thing to be grasped for" indicates, in the Greek, that it was not something he already had.

Bart Ehrman makes an argument in How Jesus Became God that Paul thought Jesus was an angel who had taken human form and then been adopted and exalted as an equal to God after the crucifixion.

There's no authoritative answer to these questions, though. Just informed speculation.