Definitions from 2 Peter
Apostle (1.1; 3.2): One personally appointed and issued authority by Christ for first generation Christian ministry.
Holy Spirit (1.21): The third person of the Godhead sent from the Father to glorify the Son and to indwell believers (since Pentecost). 2 Peter highlights the role of the Spirit in moving men to record revelation (1.20-21).
Knowledge/gnwsi" (1.5, 6; 2.12; 3.18): Perception of information. This form of knowledge may or may not include a life-impacting aspect. Let context be the guide.
Knowledge/ejpignwsi" (1.2, 3, 8; 2.20, 21): Affective perception of information. Peter regularly uses this form of the word to suggest true, impacting knowledge of Christ and the things of God.
Secretly introduce (2.1): Surreptitiously or slyly indoctrinate. When applied to false teachers, oneshould avoid picturing a street preacher voicing overt heresy. This is muchmore subtle.
Heresy (2.1): Opinions about major doctrines that are either uninformed by or opposed to Scripture.
Denying (2.1): To turn from following Christ, deliberately or inadvertently (i.e., through heresy).
Righteousness (1.1; 2.5, 7, 8, 21; 3.13): Conformity to God’s standard of uprightness and truth.
Unstable (2.14; 3.16): Weakness due to lack of solid doctrinal grounding.
Repentance (3.9): Change of mind in the direction of willful submission to God.
Unprincipled (2.7; 3.17): Lacking biblical doctrine, evidenced in one’s loose behavior.
Bought (2.1): Purchased by Christ, freed from sin and death, born again by the Spirit. This most certainly is applied to a believer.