Christianity was enforced by law in 380 CE under Emperor Theodosius I. This completed a process started by Emperor Constantine. The selection of the religion was driven by a political need for a unifying religion and the appeal of Christianity to the general populace.

Through a series of theological debates and church councils, a specific brand of Christianity only was endorsed by the empire, known as Nicene Christianity, named for the city of Nicea where it was refined.

 Nicene Christianity 

Nicene Christianity decided Christ was the Messiah predicted by the Old Testament. But most of what the New Testament claims a fulfilled Messianic prophecy is not Messianic when read in its OT context and is often not even prophecy.

It's not even really that clear that the Old Testament has any clear or coherent concept of a messiah at all, but the verses that are interpreted that way contain a number of expectations that Jesus did not fulfill. For example, the Jewish Messiah is expected to rebuild the Temple of Solomon, return all Jews to Israel, cause the world to worship one God, establish a one world government and bring world peace.

The Jewish Messiah was not expected to die or be resurrected, not expected to be a redeemer of sins and not expected to be God or a literal "son of God." He was just a human king, a direct blood descendant of David. He was (and still is) expected to be a national hero, liberator and king, but not God and not a savior of sins or an abrogator of Jewish law.

But Nicene Christianity as we know it decided the Jews were wrong. 

It also gave us "Paul"'s version of Christianity.

This guy never met Jesus and wasn't even from his neck of the woods. The earliest Jewish-Christians, the Ebionites, called him a fraud.

When you consider his emphasis on a divine kingdom, a vision of a new world order, and his confrontation with the Temple elite, Jesus reads a bit like a revolutionary. Roman emperors didn't want another Jewish revolutionary preaching about an imminent kingdom of God that would upend the world order. But Paul’s Jesus was different. He was a god. He was one with old authoritarian Yahweh. He was usable. 

Jesus's teachings had focused on the "Gospel of the Kingdom," and centered on loving God and your neighbor and obeying God's laws, but Paul's message emphasized salvation through belief in Jesus's atoning death and resurrection. 

By including so much of Paul's message, we inherited faith in blood sacrifice rather than just commandments to spread goodwill.

Christianity was then, ultimately chosen in a calculated political move that served imperial interests in stability and control.

Early Christians had argued on the divinity of Christ, and this brand of Christianity confirmed it. It also confirmed the idea of a threefold Godhead, with Jesus's instruction to baptize "in the name of the Father and of the Son and of the Holy Spirit" in Matthew 28:19 and in Paul's letter 2 Corinthians 13:14 and there are a few references in Hebrews 1 and 2. The concept of the Holy Trinity arose within early Christianity from attempts to understand the relationship between God, Jesus, and the Holy Spirit.

The authoritarian nature of Nicene Christianity was a key reason why it was chosen and enforced. A unified, dogmatic religion would prove to be a powerful tool to impose order and ideological control. And control would be used for conquest and coerced conversion.

Spread by the Sword

When Rome adopted Paul's Jesus, Christians who believed differently were then persecuted and coerced. Other forms of Christianity were declared heresy; non-believers and dissidents were subject to legal penalties and stripped of their places of worship. Orthodox campaigns against the Cathars and Waldensians were particularly vicious, bordering on genocides.

Then there were the non-Christians to contend with. Every evangelical kid knows about the Crusades. Distributing the love of Christ with force and violence is too ironic and absurd of a concept to draw anything less than morbid intrigue. In evangelical summer camp the Crusades made conquest, forced conversions, and military occupation in the name of Jesus look cool.

While not great at making lasting territorial gains or widespread conversion of the local Muslim and Jewish populations, other crusading movements led to the violent subjugation and conversion of entire regions within Europe itself.

The Reconquista, seeking to convert Spain and Portugal led to forced conversion, expulsion of Muslims and Jews, and the establishment of Catholic rule. The persecution is one of the longest sustained military campaigns in European history lasting 770 years. Hundreds of thousands converted to Christianity under immense pressure or threat of exile.

The Northern Crusades targeted the pagan tribes of the Baltic region (including modern-day Latvia, Estonia, and Prussia) in the 12th to 15th centuries. Refusal to convert was often met with death, and therefore ultimately successful in eliminating paganism in northern Europe.

Europeans (and subsequently Americans) inherited Roman religion under force. We did not adopt it because it offered a cohesive narrative of the world or appealed to our humanity.

Enforced by the Stake

Enter the Inquisitions. The Medieval Inquisition (12th–15th centuries) began in 1184, when Pope Lucius III directed bishops to conduct local "inquisitions" (inquiries) for heresy in their dioceses. Pope Gregory IX established papal-controlled tribunals in 1231, run by traveling inquisitors with systematic procedures for investigations and trials.

If the accused didn't confess during an allotted grace period they were subjected to trial. The Church itself did not perform executions; they handed "heretics" over to secular authorities for burning at the stake. Pope Innocent IV authorized the use of torture to extract confessions, making such confessions obtained under extreme distress very unreliable.

The Spanish Inquisition (1478–1834) was established by the Catholic monarchs Ferdinand II and Isabella l after reconquering Spain from Muslim rule. They used it to enforce Catholic orthodoxy and consolidate their political power. The main target was Jews who had converted to Christianity who were suspected of secretly practicing Judaism.

Under the first Grand Inquisitor, Tomás de Torquemada, thousands were sentenced to death. The persecution eventually led to the 1492 edict that expelled all unconverted Jews from Spain. The Spanish Inquisition also targeted Muslim converts, "witches", and others deemed heretical or deviant, using public sentencing ceremonies called auto-da-fés (act of faith) which featured torture.

The Roman Inquisition of the 16th Century was responsible for the trial of Galileo Galileo where he was condemned for teaching that the Earth revolved around the sun.

We ended up with the Bible because of "Paul", Constantine, swords and stakes. We live in a world that STILL snubs science and academia because they pose threats to the popularity of Christianity. 

But we can close that book.