In the first century, no one needed to confess that Christ is homoousious with the Father. But after the fourth century, to deny the homoousious is to fall into [at least material] heresy.This is dead wrong and gets things exactly backwards. It has always been heresy to deny the Son's divinity. Arius was a heretic before Nicaea, and the Nicene council simply affirmed (with respect to Arianism) what was always the teaching of the Bible.
The church does not make up orthodoxy. When the church does its job correctly, it merely recognizes the truth that was already once delivered to the saints. There was no new delivery in the fourth century or any of the succeeding centuries.
Of course, Romanists have to put the cart before the horse, because they've added to the gospel. If they tried to claim that it was always heresy to deny the Immaculate Conception, they'd have to treat Augustine, and the Augustinians down through Aquinas as heretics. So, they place the cart before the horse and say that it is only heresy to deny the Immaculate Conception after "the Church" makes that doctrine part of the gospel.
The absurd result is the one that Bryan Cross has illustrated above, where the Son's divinity becomes something that it was ok to deny before 325 A.D.
Amazing - absolutely amazing.