Why do noveau protestants point to Constantine as the definitive break from True Christianity when clearly,?

It is also clear from the declaration of the contemporary historian of the Church that Helena, from the time of her conversion had an earnestly Christian life and by her influence and liberality favoured the wider spread of Christianity. Tradition links her name with the building of Christian churches in the cities of the West, where the imperial court resided, notably at Rome and Trier, and there is no reason for rejecting this tradition, for we know positively through Eusebius that Helena erected churches on the hallowed spots of Palestine.