In today's Gospel prologue, St. Luke mentions many members of the early Church. But he only mentions one by name: Theophilus, for whom St. Luke compiled his Gospel account. So who are the others, and why does he mention them? He mentions them in order to put his account of the Gospel within the context of the Church.
Sometimes you'll hear of Christians calling Christianity a "religion of the book", the "book" in question being the Bible. However, while the Bible lies at the heart of our Christian Faith, Christianity does have a more primary foundation, and that is the Church. Christianity is more truly a "religion of the Church" than "of the Book". To be clear: it's not the Bible, but the Church that is at the heart of the Faith. Jesus founded the Church, but He did not write the New Testament: He left that job to the apostles. The writing of the New Testament was part of the mission that began on the day of Pentecost.
Fifty-two days before Pentecost, at the Last Supper, Jesus had instituted the Holy Sacrifice of the Mass and the ordained Priesthood. All the fundamentals of our Catholic Church were already in existence decades before even the first book of the New Testament was written. It's in light of this that St. Luke the Evangelist points out in his prologue that "just as those who were eyewitnesses from the beginning and ministers of the word have handed [the events of Jesus] down" by word of mouth, so Luke himself in turn decided to write his Gospel account "down in an orderly sequence". Why does St. Luke do this? He tells us: "so that you may realize the certainty of the teachings you have received."
The Bible did not fall out of the sky, bound in leather. The Holy Spirit, who is the primary author of Sacred Scripture, chose ordinary Christians in those first decades of the Church to pass on by word of mouth the words and works of Jesus. In the twenty-first century, we're blessed to have free access to Scripture, general literacy, and study guides written by faithful Catholic Scripture teachers. Nonetheless, we should not forget that the original means of handing on the Good News was Christians sharing the Gospel by sharing their experiences of being disciples. This process of oral tradition occurred countless times before the first word of the New Testament was ever written down.
This process of the Word of God taking shape in the New Testament by means of tradition teaches us an important lesson about the Church. Being a Christian is not just about "me and Jesus". Being a Christian means recognizing that our own lives are bound up with the other members of the Mystical Body of Christ. The grace of our Lord Jesus Christ, more often than not, flows into your life through the Communion of Saints: that is, through your fellow members within the Body of Christ. This is by God's design.
God promises to give us always the grace we need to face any situation, but sometimes we refuse His grace because we refuse the means by which He wills to give us that grace. Within the body of the Church, God strengthens us through each other.