Cyclopedia of Biblical, Theological and Ecclesiastical Literature
The use of this word by ecclesiastical writers is derived from that of the jurisconsults, who employ it to designate a remission of punishment or of taxes, especially such a general amnesty as was sometimes proclaimed by an emperor on an extraordinary occasion of rejoicing. Hence the word passed into ecclesiastical usage in the sense of a remission of penalties for offences against church discipline and order.
Usually there were four stages or degrees through which offenders had to pass before regaining communion: (1) weepers, (2) hearers, (3) kneelers, (4) bystanders; and usually several years had to be spent in each. Now the bishop, according to St. Gregory, might, in proportion to their conversion, "rescind the period of their penance; making it eight, seven, or even five years instead of nine, in each stage, should their repentance exceed in depth what it had to fulfil in length, and compensate, by its increased zeal, for the much longer time required in others to effect their cure." Eventually this system was greatly extended, until it reached the abuses that provoked the Reformation. Indulgentia (indulgence), a name sometimes applied to baptism in the early Christian Church, as being attended, when blessed by the Holy Spirit, with absolution or the remission of sins. It was esteemed the most universal absolution and the greatest indulgence in the ministry of the Church.
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McClintock, John. Strong, James. Entry for 'Indulgence (2)'. Cyclopedia of Biblical, Theological and Ecclesiastical Literature. https://www.studylight.org/encyclopedias/eng/tce/i/indulgence-2.html. Harper & Brothers. New York. 1870.