v. 3 — Address, intent, and urgent appeal
The writer addresses the audience as and notes an initial desire to write about their shared concerns. He reports being compelled instead to issue an exhortation: they must for what has already been entrusted to the community.
v. 4 — Reason for the appeal
The explanation is internal: certain people have entered unnoticed. They are described as distorting moral expectations and as rejecting the community’s acknowledged authority, including “our only … and Jesus Christ.”