continued steadfastly in - “attended constantly upon.”
the apostles’ doctrine - “teaching”; giving themselves up to the instructions which, in their raw state, would be indispensable to the consolidation of the immense multitude suddenly admitted to visible discipleship.
fellowship - in its largest sense.
breaking of bread - not certainly in the Lord’s Supper alone, but rather in frugal repasts taken together, with which the Lord’s Supper was probably conjoined until abuses and persecution led to the discontinuance of the common meal.