Apostolic Pardon

What is the origin of the apostolic pardon in the Viaticuum sacrament?
And does it remit all purgatory?

I don’t know the answer to your first question, but to the second – yes, it does depending on the disposition of the person (as with any plenary indulgence).

I believe the origin of this can be found in Holy Scripture. Note what the link on footnote 7 reads.

14 Is anyone among you sick? 6 He should summon the presbyters of the church, and they should pray over him and anoint (him) with oil in the name of the Lord,

15 and the prayer of faith will save the sick person, and the Lord will raise him up. If he has committed any sins, he will be forgiven. 7

James 5:14-15 NAB

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