ISB ~ 2 Peter 3 v16

all his letters:Peter is aware that Paul wrote several epistles and is knowledgeable about their contents. Some think this mention of “letters” refers to a purportedly well-known collection of Pauline writings that was put together in the late first century. Unfortunately, there is no real evidence to confirm this hypothesis, nor is there a convincing reason why the epistles of Paul could not have been assembled earlier, even within the apostle’s lifetime. After all, Paul encouraged his Churches to share and exchange his letters among themselves (Col 4:16).

hard to understand:i.e., susceptible to misinterpretation.

the ignorant:Or, better, “the uninstructed”. The idea is that Paul’s teaching is not easily or rightly understood by those who lack sound formation in apostolic doctrine.

The false teachers denounced in the letter are a case in point:most likely, they are the primary culprits guilty of twisting Paul’s doctrine to fit the mold of their own heretical beliefs. There is no insinuation in this verse that Peter himself found the letters of Paul particularly difficult or obscure.

the other Scriptures:Indicates that Paul’s epistles were revered and credited with the same level of authority as the books of the OT. Concretely, this probably means they were being read aloud in the context of the Christian liturgy.