Constantine the Great became Roman Emperor after the battle of the Milvian Bridge on October 28, 312. Before the battle, Constantine saw a cross on the sky with the words: “In this sign you shall conquer”. In 313, Constantine issued the Edict of Milan which allowed religion freedom, abolished all laws against Christianity, entitled the Church to own properties, and made Sunday a day of rest.

Christianity attracted all people by the truth of its doctrine, the purity of its morals, and the beauty of its ceremonies. It eventually became the chief religion of the Roman Empire. In 391 Emperor Theodosius I declared Christianity the official religion of the Empire. Christianity, the religion of faith, hope, and love conquered paganism, a religion of doubt, despair, and hatred.

After winning her freedom, the Catholic Church had to combat internal conflicts of heresies. The chief heresies from the fourth to the eighth centuries were Arianism (denied the divinity of Christ), Macedonianism (denied the divinity of the Holy Spirit), Pelagianism (denied original sin and the necessity of grace), Nestorianism (taught the existence of two persons in Christ and denied Mary as Mother of God), Monophysites (denied the humanity of Christ), Monothelites (taught that Christ has only one will), and Iconoclasts (attacked the veneration of holy images).

The struggles with heresies caused Church to define her teachings through ecumenical councils. The Council of Nicaea (325) drew up a profession of faith—the Nicene Creed—- proclaiming Jesus Christ as true God and true man. The Council of Constantinople (381) affirmed the divinity of the Holy Spirit. The Council of Ephesus (431) defined the true personal unity of Christ and declared Mary the Mother of God. Under Pope Leo I, the Council of Chalcedon (451) defined the two natures (Divine and human) in Christ. The Second Council of Constantinople (553) confirmed the decisions of the first four ecumenical councils. The Third Council of Constantinople (680-681) defined the two wills in Christ, the Divine and the human, as two distinct principles of operation. The Second Council of Nicaea (787) regulated the veneration of holy images.

The chief opponents of heresies were the Fathers of the Church. The most noteworthy among the Greek Fathers are St. Athanasius, St. Basil, St. Gregory of Nazianzum, and St. John Chrysostom. Among the Latin Fathers are St. Ambrose, St. Augustine, St. Jerome, and St. Gregory the Great.

While the Fathers of the Church acted as defenders of the true Faith, the hermits and monks shone as models of penance. The hermits were pious Christians who fled from the seductive pleasures of the world, to prepare themselves in solitude, by prayer and self-denial, for a happy death. St. Paul of Thebes (+340) was the first hermit who had fled a persecution under Emperor Decius (249-251). St. Anthony (+356) built the first monastery, and was called the Patriarch of Monks. After the death of St. Anthony, one of his disciples, St. Athanasius, wrote his life. The biography of St. Anthony projected the ascetic ideal and peopled the desert with monks. Through St. Anthony, the solitary life gave rise to the monastic life.

There were two kinds of monastic life: the eremitical or hermit life and the cenobitical or common life. St. Anthony and his disciples formed a community of eremitical life: they lived on their own and came together mainly to partake in the Sunday liturgy. St. Pachomius (+345) was one of the principal founders of cenobitic monasticism. The monks lived in community, kept scheduled times of prayer together, and took weekly turns at various tasks. The cenobitic form of monasticism was perpetuated in the East by St. Basil (+379), and in the West by St. Benedict (+547).

公元312年十月二十八日，羅馬君士坦丁大帝在一場以寡敵眾的戰爭前，忽然他看見天空出現一個十字架，旁邊寫著「以此標記，你當戰勝！」果然，米蘭橋一役，君士坦丁獲得全勝；於是，他馬上頒佈「米蘭法令」，廢除對天主教禁教的一切法律及規條，容許天主教自由傳教、擁有物業權，並切定星期日為全民休息日。

基督聖教的真道、高尚的操守和入勝的禮儀，吸引眾多人民皈依，轉眼成了羅馬帝國的主流宗教信仰。公元391年，在皇帝提奧杜塞一世宣佈下，基督聖教成了這幅原廣大的帝國的國教。基督聖教的信、望、愛德，征服了異教的猜忌、絕望和仇恨。

獲得自由的天主教，不久要面對內部而來的異端。公元四世紀至八世紀，教會內出現一些異端：「亞略主義」（否定基督的天主性）、「馬奇道主義」（否定聖神的天主性）、「白拉奇學說」（否認原罪及聖寵的重要）、「聶斯多略派（景教）」（主張基督有兩位格及兩性體，瑪利亞只是基督人性的母親，而非天主之母）、「基督單性說」（否定基督的人性）、「基督單一意志說」（主張基督只有一個意志，否認基督兼有神、人兩意志），和「反對聖像派」（宣稱供奉聖像不合教會精神）。這連串的內鬨異端，使聖教會召開幾次大公會議，以澄清和釐定當信教義。

公元325年，尼西亞大公會議，擬定「尼西亞信經」，宣認耶穌基督是真天主亦真人。公元381年，君士坦丁堡大公會議，確認聖神的天主性。公元431年，厄弗所大公會議，定明基督只有一位格，遂公認瑪利亞是天主之母。公元451年，加采東大公會議，教宗聖良一世聲明，基督是神人兩性一位。公元553年，第二次君士坦丁堡大公會議，重新確認和確立先前四次大公會議頒佈的當信道理。公元680-681年，第三次君士坦丁堡大公會議，譴責「一意志」論，聲明基督神人兩性、兩志，清淅兼備。公元787年，第二次尼西亞大公會議，規定要尊敬聖像。

早期聖教會教父們，斥異衛道不遺餘力，為人樂道推崇的，有東方禮儀的聖師亞大納削、聖師巴西略、聖額我略納齊盎和聖若望金口。拉丁禮儀的有聖師盎博羅削、聖師奧斯定、聖師熱羅尼莫和教宗聖額我略一世。

同一時期，教會有很多棄俗潛修、克己行苦的聖賢，樹立祈禱修德、棄絕自我、獨隱避世，以求善終的表樣。當中首先獨修聖保祿 (公元+340年)，為逃避羅馬皇帝戴西于迫害聖教，跑到曠野獨居隱修。與前者惺惺相惜的，是聖安當（公元+356年），他修建了第一座隱修院，被譽為「隱修士的祖宗」，而他的弟子聖亞大納削記下他的生平，昭示了在曠野的苦行靜修的實況，彰顯了獨居隱修的隱士生活。

其實，隱修生活有兩種：獨居及群居。聖安當偕同他的弟子，都是獨居隱修，只在主日聖儀才聚在一起。聖巴各滿（公元+345年）則成立群居的隱修團體，修士們共同一起生活，在特定時間集體祈禱，並共同分擔院務；群居的隱修制度，在東方禮儀的聖教中，被聖巴西略（公元+379年）推廣，在拉丁禮儀的聖教中，被聖本篤（公元+547年）推廣。