Christianity

Christianity is the religion founded by Jesus Christ and spread by his Apostles and later followers.

It is a monotheistic religion centered on the life, teachings, death, and resurrection of Jesus Christ, who is regarded as the Son of God and the Saviour of humanity. Christians believe in the Holy Trinity—Father, Son, and Holy Spirit—as one God in three persons. The faith is based on the Bible, which includes the Old and New Testaments, with the New Testament detailing the life of Jesus and the teachings of his apostles. Central to Christianity is the belief in salvation through faith in Jesus Christ, who died to atone for the sins of humanity and offers eternal life to believers. Christianity emphasizes love, compassion, and forgiveness, and it has various denominations, the most accurate being the Reformed tradition.

Beginning with the early church established by Jesus Christ and his apostles, over time, the purity of the gospel was obscured by the rise of doctrines and practices within the Roman Catholic Church that strayed from biblical teachings. The Protestant Reformation, which began in the 16th century with figures like Martin Luther and John Calvin, returned the church to its scriptural roots.

This helped to the define the Five Solas of Protestantism, a key tenant of most protestant Christians. They are as follows:

Sola Scripture: Scripture is the only source of normative, apostolic, infallible revelation and that “all things necessary for salvation and about faith and life are taught in the Bible with sufficient clarity so that the ordinary believer can find it there and understand it.”
Sola fide: Justification is by faith alone, meaning that individuals are declared righteous before God solely through faith in Jesus Christ, not by any works or human effort. This doctrine asserts that faith in Christ's atoning sacrifice is the only means by which sinners can receive forgiveness and be reconciled to God, with good works being the fruit and evidence of genuine faith, not the basis of salvation.
Sola gratia: Salvation is by grace alone, emphasizing that it is entirely the work of God's unmerited favour, not dependent on human effort or merit. This doctrine teaches that humans are utterly incapable of contributing to their own salvation due to their sinful nature, and it is only by God's sovereign grace that they are redeemed and brought to faith. Grace is understood as a gift from God, freely given, and the sole cause of a believer's justification, regeneration, and ultimate salvation.
Solus Christus: Christ alone is the sole mediator between God and humanity, and salvation is found exclusively through His person and work. This doctrine asserts that Jesus Christ's life, death, and resurrection are the only sufficient means by which sinners can be reconciled to God.
Soli Deo gloria: Glory to God alone, asserting that all of creation, including salvation, exists for the purpose of glorifying God. This doctrine teaches that God is the ultimate source of everything and deserves all honour and praise, with no credit due to human beings or any other entity.

Origins

Christianity emerged in the first century out of the fulfilment of Second Temple Judaism and the Law of Moses with the crucifixion of Jesus Christ. Prior to this, the Jewish holy text that we now know as the Old Testament spoke of a Messiah who would be sent by God to redeem Israel and establish God’s kingdom. This Messiah, according to Jewish prophecy, was expected to be a descendant of King David, a figure who would restore Israel, bring peace, and lead humanity into a renewed relationship with God.

These prophecies can be found in:

Isaiah 53, which describes a “suffering servant” who would bear the sins of many, a foreshadowing of Jesus’s sacrificial death.
Micah 5:2, which predicts that the Messiah would come from Bethlehem, the birthplace of Jesus.
Zechariah 9:9, which describes a king arriving humbly on a donkey, which aligns with Jesus’s triumphal entry into Jerusalem.

Jesus was born to the Virgin Mary at the start of the first century. Growing up in Nazareth, he began his public ministry around the age of 30, teaching about God’s kingdom, performing miracles, and affirming that he was the long-awaited Messiah. He was crucified under Roman governor Pontius Pilate for claiming to be the Son of God, a statement considered blasphemous by religious leaders, such as the Pharisees and Sadducees. He rose from the dead on the third day, offering forgiveness and eternal life to all who place their faith in him.

Following this, the Apostles gathered in the Temple in Jerusalem, where Jesus, who they had just seen ascend to heaven, reappeared to them to tell them to prepare for the coming of the Holy Spirit. Acts 2 describes the coming of the Holy Spirit on the day of Pentecost. Suddenly, a sound like a mighty rushing wind filled the place, and what appeared to be tongues of fire rested on each of them. They were filled with the Holy Spirit and began speaking in various languages, enabling them to share the message of Jesus with people from diverse regions who were in Jerusalem for the feast. This event marked the birth of the Christian Church, as the Apostles boldly proclaimed Jesus as the risen Messiah, leading to the conversion and baptism of about 3,000 people that day.

The Apostles spread Jesus' teachings across the globe, and all of them were martyred, save for the Apostle John.

The Council of Nicaea

Following the deaths of the Apostles, their disciples continued to affirm their beliefs despite persecution from various Roman emperors, mostly notably the Emperor Nero. Three of the most influential of these so-called "Apostolic Fathers" were Clement, the Bishop of Rome, Ignatius, the Bishop of Antioch and Polycarp, the Bishop of Smyrna. These three leaders defended the faith against heresies, encouraged steadfastness in persecution, and reinforced the doctrines handed down from the Apostles, emphasizing God's sovereignty, the authority of Scripture, and the centrality of Christ’s redemptive work. Their writings and martyrdoms became foundational for the early Church.

Despite these valiant efforts, heresies soon arose. Notable early heresies include Docetism, the belief that Jesus only appeared to be human and did not have a physical body, Marcionism, the belief that the God of Judaism and Christianity were in fact two separate gods, one evil and one good, and eventually Gnosticism, which taught that salvation came through secret knowledge and that the material world was irredeemably corrupt.

Irenaeus, a prominent early Church Father and Bishop of Lyons, vigorously opposed Gnostic teachings in his work Against Heresies. He argued that true knowledge comes from the Apostolic teachings preserved by the Church, emphasizing that salvation is available to all through faith in Jesus Christ, who fully entered the material world to redeem it. Irenaeus defended the goodness of God’s creation, the reality of Jesus’s incarnation, and the universal nature of salvation, reinforcing orthodox Christianity and helping to establish the doctrinal foundations of the early Church against Gnostic influences.

Radically unbiblical heresy had lost any influence it had by the beginning of the 4th century, but nontrinitarian heresies were the next to arise. Trinitarianism had been a central doctrine of Christianity since the time of the Apostles, and yet many had forgotten and fallen into a state of denial. One of these deniers was the heresiarch, Arius. Arius taught that Jesus Christ, while divine, was not equal to God the Father but was instead a created being, distinct in essence and subordinate to the Father. This doctrine, known as Arianism, claimed that "there was a time when the Son was not," directly opposing the Church's belief in the co-eternity and co-equality of the Father, Son, and Holy Spirit.

When Roman Emperor Constantine converted to Christianity in 312 CE, he organised the Christian Bishops across the Mediterranean and beyond to meet at the Greek city of Nicaea, for what would become known as the Council of Nicaea. Amongst those in attendance, there was not a wide variety of views and the vast majority were in favour of a Trinitarian interpretation of scripture. Notable adherents of the Orthodox view included Hosius of Corduba, Alexander of Alexandria, Athanasius of Alexandria and Acesius of Constantinople. Arius and his small following amongst some unorthodox bishops were easily refuted and declared as heretics. It was at this council that the Nicene Creed was written. All but three of the over 300 bishops in attendance signed the creed and subscribed to it's doctrine.

Arius was exiled to Illyrium for a short time, where he would spread his heresy amongst Roman officials there. This included Constantine's sister, Constantia, who urged Constantine to support Arius and to persecuted the orthodox bishops. Constantine lifted Arius' exile and attempted to force the church to readmit him, however Arius died before this could happen. Constantine met with other Arian theologians, such as Eusebius of Nicomedia, who successfully swayed him in favour of Arianism. Constantine would be baptised by Eusebius shortly before his death in 337 CE, effectively symbolising his siding with the Arians.

In the east, Constantine was succeeded by the Emperors Constantine II and Valens, two Arians. Despite Arianism being a minority, this gave them much more political power which they used to try to stamp out Christianity. A much later Church Father, Jerome of Stridon, described this time as when "the whole world groaned and was astonished to find itself Arian."

However, the power of Arians was subverted by the zealous Athanasius of Alexandria, who had been in attendance during the Council of Nicaea. He became popular with his charisma and his intellect. Through his many writings and protests, he was able to stop Christianity from compromising it's beliefs and giving in to the Arians. This led to the Arians attempting to consolidate political power to win more converts amongst the bishops, but before they were able to, they turned to infighting and destroyed themselves. The Council of Constantinople was held in 381 CE, which reaffirmed the orthodox trinitarian view and the Nicene Creed.

The Councils of Nicaea and Constantinople were two of the so-called "ecumenical councils". Today, most Calvinists affirm four of these councils.

Council of Nicaea (325 AD): Affirmed the full divinity of Jesus Christ and condemned Arianism, establishing the Nicene Creed and the doctrine of the Trinity (the Son is of the same essence as the Father).
Council of Constantinople (381 AD): Reaffirmed the Nicene Creed, affirming the full divinity of the Holy Spirit and clarifying the doctrine of the Trinity as equally one in essence and three in persons.
Council of Ephesus (431 AD): Affirmed the title "Theotokos" (God-bearer) for Mary, defending the truth of Christ's full humanity and divinity against Nestorianism, which sought to separate the two natures of Christ.
Council of Chalcedon (451 AD): Defined the doctrine of the hypostatic union, asserting that Jesus Christ is fully God and fully man, with two distinct natures united in one person, opposing both Monophysitism and Nestorianism.

The Protestant Reformation

Roman Catholicism originated with Leo, the Bishop of Rome, who declared that the Church of Rome was the supreme authority in Christendom. This claim was based on the pseudohistorical belief that Peter the Apostle had been the first bishop of Rome and that his authority had been passed down to his successors. Over time, the Church of Rome, later called the papacy, grew in power, and the Bishop of Rome became recognized as the Pope, the head of the worldwide Church. This development, along with the consolidation of key theological doctrines and liturgical practices, solidified the structure of the Catholic Church as the central institution of Christianity in the Western world.

In the early 16th century, the Catholic Church was criticized for its practices, particularly the sale of indulgences, which led to the Protestant Reformation. Martin Luther, a German monk, sparked this movement in 1517 by posting his 95 Theses, challenging the Church's authority and doctrines, especially the idea that salvation could be bought or earned through works. Other reformers, such as John Calvin and Ulrich Zwingli, joined in the effort to restore what they believed was the true gospel, emphasizing salvation by grace alone, through faith alone, and the authority of Scripture over Church traditions. These reforms led to the formation of various Protestant denominations that rejected papal authority and sought a return to biblical principles.