Read the whole passage from Newman -- though I don't think he went into corn gods as such, he does mention the idea of an incarnate god coming from India, the idea of baptism being of pagan origin, etc. etc.
A big part of the "free-thinker" movement of the 19th century ridiculed Christianity as merely another corn god myth.
Newman was hardly ridiculing Christianity but rather pointing out the origins of what the Catholic Church was practicing under the rubric of Christianity.
The pagan practices may have been brought into the Catholic Church but the Christianity of the Scriptures was a God given revelation
And at the same time saying it didn’t matter as bringing such pagan teaching and practice into the Catholic Church some how “sanctified” it.
In this Newman was quite in line with The Catholic Encyclopedia quote I posted.