By “first Christians,” do you mean the first ones after the apostolic era, or do you mean the apostles themselves?
The apostles, of course, were recipients of new revelation from God. Their teachings were not dependent on culture, tradition, or Old Testament patterns. They laid out what God wanted for his church, and they did so infallibly with divine trustworthiness.
The Early Church Fathers, on the other hand, were simply trying to transmit what they had learned. As Ignatius said in one of his letters, (I’m paraphrasing from memory here), “I do not speak to you as John and Paul. They were apostles, I am not.” This explains why there is such a wide variety of ideas and positons in the fathers. As they sought to interpret and apply the apostolic tradition, inevitable differences arose. Some of these were due to cultural influences.
Regarding immersion, it shows up early. So does sprinkling. So does naked baptism, and trine baptism (three immersions), and giving the convert milk and honey or salt after the baptism. I doubt that the variety is of apostolic origin. Instead, I think it is just folks trying to make the ceremony more elaborate and pleasing to the senses.
You may remember that one of the reasons for the East-West schism was that the West (Rome) was baptizing by one immersion, instead of three “as the apostles taught” (according to the Greeks).
Although immersionists often appeal to the Greek word for proof (baptizo), the real strength of their argument stems from theology: what does baptism signify? If it symbolizes burial and resurrection, immersion follows naturally. If it symbolizes something else, then other modes might fit better.
All of this reasoning assumes that the New Testament doesn’t speak clearly regarding the mode of baptism. And, quite obviously, immersionists will never concede that.
