Iustitia Dei Part 5: Justification and the Sacraments

"In conclusion, it may be stated that the medieval period saw that justification of the sinner firmly linked to the sacramental life of the church, a sound theological link having been established between justification and the sacraments. This linking of justification to the sacramental system of the church has profound theological and pastoral consequences, of which the most important is the tendency to assert iustificatio extra ecclesiam non est. Although the theologians of the medieval period were aware that God was not bound by the sacraments, the tendency to emphasise the reliablity of the established order of salvation, of which the sacramental system is part, can only have served to convey the impression that the sinner who wishes to be reconciled God must, de facto, seek the assistance of a priest"

- Alister McGrath pg. 127 of Iustifitia Dei