A tidbit from the ancient Jewish philosopher Philo (Special Laws 1.186-187):
(186) On the tenth day the fast takes place which they take seriously–not only those who are zealous about piety and holiness, but even those who do nothing religious the rest of the time. For all are astounded, overcome with the sacredness of it; in fact, at that time the worse compete with the better in self-control and virtue. (187) The reputation of the day is due to two reasons: one that it is a feast and the other that it is purification and escape from sins for which anmesty has been given by the favors of the gracious God who has assigned the same honor to repentance that he has to not committing a single Sin.
Translation is from Yonge and the emphasis is mine. I noticed that this passage equally amused Menachem Mendel, who posted on it here. There was, of course, no Kol Nidre in antiquity (it emerged in the early Middle Ages), and Philo says nothing about a synagogue (the picture above is a much later Alexandrian synagogue). Still, it is worth thinking about how the Alexandrian Jewish community in the first century CE might have approached the Day of Atonement.