Ignatius of Antioch (d.108) is counted among the Apostolic Fathers, those who had direct connection to the Apostles or were taught second-hand by someone connected to them. In other words they represent the earliest testimonies of Christian leaders in the period just after the New Testament was completed (around the year 95).
Ignatius was the Bishop of Antioch, the city where the disciples were first called 'Christians' (Acts 11:26).
It's noteworthy that in Clement's letter to Corinth (written in the late 90's) the terms 'bishop' and 'presbyter' (or elder) are used synonymously as seen in the New Testament. In contrast, Ignatius pushes hard to make a distinction between the offices, marking the early stages of episcopacy or rule of bishops, the hierarchical form of Church government that would emerge in subsequent centuries. It's safe to say that Ignatius had no intention of subverting the teaching of the apostles. Rather, he sought to protect it. The Church during this time was under assault as false apostles were everywhere making extravagant claims regarding their teaching - we see hints of this already in the pages of the New Testament.
Ignatius wanted to demonstrate a pedigree of true doctrine connected to those who had been taught in succession, a chain leading back to the apostles themselves. This would over time turn into the notion of Apostolic Succession, a doctrine of great importance to Roman Catholics, Eastern Orthodox, Anglicans, and others. Historically most Protestants have argued the mantle of succession belongs to those who follow the teachings of the apostles as opposed to a succession of rituals and forms. Sadly such rites (and the claims associated with them) would become corrupted and hollow over time.
Ignatius is best known for his epistles and yet the history of these texts and manuscripts has been greatly disputed. Basically the most trustworthy are the shorter recensions, and letters testified to by the historian Eusebius in the 4th century. There are longer (and somewhat dubious) manuscripts as well as some other letters claimed to have been written by Ignatius but are likely spurious.
Said to have been one of the children blessed by Jesus, tradition also suggests that Ignatius was also hand-picked for office by the Apostle Peter. Condemned to death for his refusal to deny Christ, Ignatius was sent to Rome where according to tradition he was devoured by lions in the Colosseum.
As he made the journey along with Roman guards (the ten leopards he refers to in his letters), he was permitted to visit congregations in Asia Minor. After his departure he wrote letters to the churches at Ephesus, Magnesia, Tralles, Philadelphia, Smyrna, and Rome. And finally there is a letter to Polycarp, the Bishop of Smyrna and another famous Apostolic Father and martyr.
Ignatius clearly testifies that the Early Church believed in the Deity of Christ and the value and efficacy of the Eucharist (or Communion), calling it the 'medicine of immortality'. He warned against the Judaizers who were trying to compel Christians into keeping the Sabbath. And as stated he is considered the earliest witness to a gradation in Church government, the distinction between the office of Bishop and Presbyter. He lays great emphasis on the role and importance of the (Episcopos) Bishop in governing the affairs of the Church and guarding its orthodoxy. We also see the term 'catholic' emerge with Ignatius, the idea of Christianity, a universal concept that binds all individual Christians and all churches together. This is not the same as Roman Catholicism which finds that universality and unity in its connection with and obedience to the Bishop of Rome, who would in time become known as the Pope. This would emerge later and while Ignatius laid some of the groundwork for it, it is unfair to blame him for what came after.
Ignatius (who called himself Theophorus or God-bearer) puzzles modern Christians in his seeming zeal to die a martyr. He comes across as almost fanatical but it demonstrates a very different mindset and posture toward this world. We can learn a great deal from Ignatius and his letters provide a fascinating look at the ancient Church. His many quotations provide an early testimony to the fact that the gospels and many New Testament epistles were known and treated as Holy Scripture.