Justin Martyr, a converted Greek philosopher, addressed his First Apology to Emperor Antoninus Pius around 155 AD. It defends Christianity against pagan slanders, describes a typical Sunday service in detail (Scripture reading, sermon, Eucharist, almsgiving), and quotes from the "memoirs of the apostles" — the Gospels — as authoritative scripture.
Why this matters
The earliest surviving description of a Sunday Christian service, written within ~120 years of Pentecost. Confirms the order of worship, the use of the Gospels in the liturgy, and the centrality of the Eucharist long before Constantine.
Scripture references
Acts 2:421 Corinthians 11:23-26Hebrews 10:24-25
Location
Various