In the first few centuries of Christian history, Christians found themselves persecuted simply because they were Christian. While there are debates about the number of Christians that were actually tortured or killed, the fact remains that early Christians were tortured and killed, and Christian apologists wrote to authorities — from local governors to the emperor himself — decrying the lack of religious liberty that permitted this sad state of affairs. Christians argued that no one should be compelled to follow a particular set of religious beliefs, and that everyone should be free to choose what to believe in religious matters as long as their actions did not pose a threat to the empire. That is, they acknowledged that if a particular religion actually promoted a particular evil such as rape, murder, or regicide, there would be a good reason to hinder its activities. But, the apologists continued, Christianity promoted the common good and Christians should be given their religious liberty because they provided positive contributions to the state. Thus, for example, Athenagoras, in his apology written to Marcus Aurelius and Commodus, wrote:
Names are not deserving of hatred: it is the unjust act that calls for penalty and punishment. And accordingly, with admiration of your mildness and gentleness, and your peaceful and benevolent disposition towards every man, individuals live in the possession of equal rights; and the cities, according to their rank, share in equal honour; and the whole empire, under your intelligent sway, enjoys profound peace. But for us who are called Christians you have not in like manner cared; but although we commit no wrong — nay, as will appear in the sequel of this discourse, are of all men most piously and righteously disposed towards the Deity and towards your government — you allow us to be harassed, plundered, and persecuted, the multitude making war upon us for our name alone.
Christians, Athenagoras complained, were treated unjustly simply because they were Christian and not due to anything they had done. While hearsay and false accusations had condemned the name “Christian,” he argued that justice should stand on actual evidence of wrongdoing. Christians were good citizens, and by harming them, the empire was harming itself. Christians should have equal rights to others, and should be free to believe and worship as they choose as long as their actions remained just and good. If a particular Christian did evil, they should be tried for what they have done, not for their Christianity. It was, Athenagoras explained, what the empire normally allowed for others: “In your empire, greatest of sovereigns, different nations have different customs and laws; and no one is hindered by law or fear of punishment from following his ancestral usages, however ridiculous these may be.”
Earlier, Tertullian made a similar plea. He argued that rulers should learn what Christians believe and do rather than heeding rumors that suggested all kinds of evil deeds they perpetuated. If authorities did this, they would learn about the good things Christians do. Which is why, he suggested, Christians should be given religious liberty like everyone else, the freedom to believe and worship as they wish:
However, it is a fundamental human right, a privilege of nature, that every man should worship according to his own convictions: one man’s religion neither harms nor helps another man. It is assuredly no part of religion to compel religion — to which free-will and not force should lead us — the sacrificial victims even being required of a willing mind. You will render no real service to your gods by compelling us to sacrifice. For they can have no desire of offerings from the unwilling, unless they are animated by a spirit of contention, which is a thing altogether undivine.
God, or the gods (for those who believed in a plurality of them) were better respected and followed when people worshiped them by their own choice and with their own will. It should be left to the gods to decide the fates of those who do not do so. There was something demonic, Tertullian thought, in trying to force people to believe and worship and sacrifice to the gods against their will:
But as it was easily seen to be unjust to compel freemen against their will to offer sacrifice (for even in other acts of religious service a willing mind is required), it should be counted quite absurd for one man to compel another to do honour to the gods, when he ought ever voluntarily, and in the sense of his own need, to seek their favour, lest in the liberty which is his right he should be ready to say, “I want none of Jupiter’s favours; pray who are you? Let Janus meet me with angry looks, with whichever of his faces he likes; what have you to do with me?” You have been led, no doubt, by these same evil spirits to compel us to offer sacrifice for the well-being of the emperor; and you are under a necessity of using force, just as we are under an obligation to face the dangers of it.
The fourth century Christian writer Lactantius, who served as a link between the time of the persecutions and Constantine, continued to advocate for such a standard for the state. He wrote that religion promoted by bloodshed and force only profanes religion itself:
For if you wish to defend religion by bloodshed, and by tortures, and by guilt, it will no longer be defended, but will be polluted and profaned. For nothing is so much a matter of free-will as religion; in which, if the mind of the worshipper is disinclined to it, religion is at once taken away, and ceases to exist. The right method therefore is, that you defend religion by patient endurance or by death; in which the preservation of the faith is both pleasing to God Himself, and adds authority to religion.
Other apologists exhorted public officials to stop their persecution of the Christians. They presented a common thread of thought. The state, and its leaders, should promote justice. If some injustice is found, they can and should put a stop to it. But the problem was that the state itself committed an injustice when it forced Christians to act against religious liberty. Now, as Romans often knew little to nothing about the beliefs and practices of the Christian faith, it was easy for gossip to spread suggesting all kinds of evils being perpetuated by Christians. This is why much of the writings of the Christian apologists focused on dealing with such false claims. When Christians were shown to be morally sound, however, the argument stood that they should be given liberty to practice their faith as they wished. St. Justin, in his First Apology, made that case: “And if these things seem to you to be reasonable and true, honour them; but if they seem nonsensical, despise them as nonsense, and do not decree death against those who have done no wrong, as you would against enemies.”
Religious liberty was at the forefront of early Christian struggles with various authorities in Rome. The struggle bore fruit under Constantine. With the Edict of Milan, Constantine proclaimed what the Christian apologists before him proclaimed: the state should give religious liberty, not just to Christians, but to everyone of good will. As Eusebius recorded, Constantine decreed:
Since this has been granted freely by us to them, your devotedness perceives that liberty is granted to others also who may wish to follow their own religious observances; it being clearly in accordance with the tranquility of our times, that each one should have the liberty of choosing and worshiping whatever deity he pleases. This has been done by us in order that we might not seem in any way to discriminate against any rank or religion. 
While after the Edict of Milan, Constantine might not have perfectly followed its ideals, we must recognize that its principles originated with the Christian apologists, and that it represents the early Christian sentiment concerning religious liberty. That said, when Christians gained power in the state, many found this power to be a great temptation, and Christian history shows a subsequent (but not complete) decline from the early Christian sentiment on religious liberty. This does not remove the fact, however, that from the foundation of the Church, Christians actively promoted religious liberty. They grounded it upon both ethical and religious principles. These early Christians believed that compulsion in religion hindered people’s ability to act in good faith.
Once Catholics today recognize the early Christian roots of the principle of religious liberty, then we can recognize what was taught by the Second Vatican Council in the documents Dignitas Humanae and Nostra Aetate relies upon ancient Christian teaching, and is a very traditional approach. This is an approach that the earliest Christians would have recognized as the one they themselves promoted. AS long as Christians did not have political power, they were not tested by the temptations faced by those in power; once they got it, it is clear many Christians fell under its sway and abused their power through the centuries. One of these abuses was to deny religious liberty to many who deserved it. At the time of Vatican II, Christendom was practically at a dead-end, and the Council Fathers were able to restore our proper Christian heritage concerning religious liberty, while criticizing any and all who would oppose it, saying it runs contrary to the teachings of Christ and the holy Apostles:
The Church reproves, as foreign to the mind of Christ, any discrimination against men or harassment of them because of their race, color, condition of life, or religion. On the contrary, following in the footsteps of the holy Apostles Peter and Paul, this sacred synod ardently implores the Christian faithful to “maintain good fellowship among the nations” (1 Peter 2:12), and, if possible, to live for their part in peace with all men, so that they may truly be sons of the Father who is in heaven. 
Religious liberty is not new, nor is it a heretical idea. Religious liberty was a fundamental teaching of the earliest Christians. They recognized the need for religious liberty. They knew God did not want slaves who acted out of coercion instead of love; God wants to be followed in and through love, for he himself is Love. Jesus said we should no longer look to be slaves, but friends and children of God (cf. Jn. 15:15). The elimination of religious liberty makes it hard for many of us to properly align ourselves with God, to see his love for us, and to effectively love him back. As Tertullian said, there is no true service to God, so long as anyone follows him merely because they are compelled to do so by others. Religious liberty, therefore, not only guarantees the right for people in society to worship as we please, but it guarantees the value of that worship, allowing us to do it truly out of our loving devotion to God.
 Athenagoras, “A Plea for the Christians” in ANF(2): 129.
 Athenagoras, “A Plea for the Christians,”129.
 Tertullian, “To Scapula” in ANF(3):105.
 Tertullian, “Apology” in ANF(3):41.
 Lactantius, “Divine Institutes” in ANF(7):157 (Book V. Chap XX).
 St. Justin Martyr, “First Apology” in ANF(1): 186.
 Eusebius, “Church History” in NPNF2(1): 379.
 Nostra Aetate. Vatican translation. ¶5.
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Henry Karlson was raised a Southern Baptist, and became a Byzantine Catholic in 1995. He is the author of The Eschatological Judgment of Christ from Wipf and Stock and writes at "A Little Bit of Nothing" on Patheos.