Justin was born at the beginning of the second century, about AD 110, so he would have been a second generation Christian, being taught by the people who knew the apostles. He was a pagan philosopher but converted when he realised that the church contained the truth that he was seeking all along.
Justin leaves us the earliest record of an actual Eucharistic service and what is interesting is that it is almost identical in form to the Mass we celebrate today. The basic structure has not changed and indeed many of the prayers, particularly the Eucharistic prayers would not seem out of place in today's Mass. He also left us a record of Baptism and again much of the service is similar to that which we have today.
At the time Justin was alive, the early Church was just finding its feet and many people were suspicious of it, not understanding what it taught. Justin wrote several works explaining the teachings of Christ but he was still martyred in about AD 165, when Marcus Aurelius was emperor.