You are correct in saying that the statement is reactive, as is most of our doctrine. Had there never been a heretic trying to lead people away from the true faith, we would still likely have a very simple understanding of our Lord like that of Ignatius. But as heretics crept in and challenged the faith of the apostles, the fathers had to spell out the limitations of that which was true and that which was not.
The beauty of this first line in the creed, is that it not only achieves this limiting goal of what is true, but it also contextualized our relationship to God, and emphasizes first that He is a father (in fact The Father) and secondly that He is the Almighty. This is not to say that His Omnipotence is secondary to His being a Father, but rather contrasts directly against Greek philosophy. Athanasius, who was a key figure in the formation of the Nicene creed, was he who freed Christianity from the shackles of Greek philosophy. It was the Greeks who believed in many gods most of which were vindictive and brute authoritarians, toying with the human race for their own fancy, but the creed presenting the God above all else, who loves His creation like a father loves his son.