In this classic study from 1899 J.W. Hanson went in detail with the belief in universal salvation in early Christianity. According to Hanson, universal salvation was the prevailing doctrine of the early Church.
For a more up-to-date study see Ilaria Ramelli’s The Christian Doctrine of Apokatastasis (2013).
Download as pdf: Universalism The Prevailing Doctrine Of The Christian Church During Its First Five Hundred Years (1899)