It is said that the devil, first, attacked the early church from without through persecution. But failing in this he changed his strategy. This time he attacked from within through heresies. The saints fought the heresies; while Holy Mother church protected her flock through Ecumenical Councils.
By the 3rd century, the church existed in relative calm and in some parts, even in luxury. There was much time for everything specially for speculative theology. Instead of being concerned with how to live holy lives, people were speculating on the nature of God. The Hierarchy begun to enter into politics and Emperors begun to manipulate the hierarchy. The church became splintered into several groups.
St. Jerome observed: that one day the whole world woke up to find itself Arian, a heresy propagated by the priest, Arius, and backed up by a bishop, Eusebius.
Before the church could win her battle against the heresy of the priest Arius, a bishop, Nestorius, arose with another heresy denying that the Blessed Virgin was the Mother of God. This was followed by the heresy of the monk Eutyches. As usual God raised champions to refute the heresies; the church summoned councils to protect the faithful. The Council of Nicea to expose the heresy of Arius, the Council of Ephesus to expose Nestorius and the Council of Chalcedon to expose Eutyches.
In fighting heresies, St. Irenaeus gave the cardinal rule: if anyone wants to know with certainty what is true in religious matters, he should study the teachings of the church written by a line of holy bishops going back to the apostles, i.e. the Fathers which are in the possession of the Catholic Church.
With heresies, the church needed no attacks from without. The enemy was now within; from her own bosom, among her own priests, her own bishops and her own religious.
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