Complex sentence: Ambrose in particular stated that the emperor should imitate David in the scale of his repentance as he had imitated him in the scale of his guilt, and excommunicated him until he did, only readmitting him to the Eucharist only after several months of public penance.
Split into simpler sentences: Ambrose in particular stated that he should imitate David in the scale of his repentance as he had imitated him in the scale of his guilt, and excommunicated him until then. Ambrose readmitted the emperor to the Eucharist only after several months of public penance.