"One was justified by their faith, their profession of allegiance to Christ, in the rite of baptism: there is no conflict between the two, rather they are an integrated whole. This goes a long way to explaining why some of the 'quirks' of the earliest church exist, such as why catechumens were considered 'saved' if they died in martyrdom before baptism: it isn’t that baptism became a proto-Pelagian 'work,' but rather that it was considered the moment of saving faith through the work of the Spirit."
For a primary source documenting this, I found this in St Hippolytus' Apostolic Tradition (while looking for something entirely different, naturally):
Note here the close connection between baptism and justification, as if one is the cause of the other.
