Religious Grandmother's Abbreviation Cipher Deciphered on the Web

Back in 2014, a certain JannaK posted what looks like an enciphered message wirtten on an index card by Grandmother (1927-1996), who lived in Minnesota. It begins with:
PDGNHBOBVPNSNHANAOENCNANHPNCPND,NUOCP
NNPNAPNMSMDKBMLPOWP,NAP,]NEENTGBTMLS
HHSSSTMALHFFTMOFPANSTP,NIOOIPNTPNROA
NTRSANTOTTAPOD,PLUADONNOANPBOOLL,PKUA
SASIMAB,PAGA,]PMOMTAVMAJAMMSSSLLAB
....

Thirteen minutes later, harperpitt pointed out that the last AAA on the front side may be "Amen, Amen, Amen." Then, TYAGF appearing earlier may be "Thank you Almighty God for..."
The back side begins with OFWAIHHB, which jessamyn pointed out is used as user names by many people and should be something well-known. Then, harperpitt came up with an idea that OFWAIHHBTN is "Our Father who art in Heaven, hallowed be thy name." This led to revealing the meaning of many phrases. The very frequent PST turned out to be "Please see that ...."
This is not a cipher in the usual sense, but initialism. I didn't know such a system is used for a text of some length until I read Chapter 13 of Elonka Dunin and Klauss Schmeh's book Codebreaking (see another post), where the system is called "abbreviation cipher".

References:
Decoding cancer-addled ramblings
"おばあちゃんが最期に残し20年間も未解決だった謎の暗号がネットの集合知によって爆速で解明へ"