Peter knew what it meant to fall from his own steadfastness. The man who first identified Jesus as the Son of God (Mt 16:6) rebuked Jesus for teaching that He’d come to die (Mt 16:21-23). The one Jesus announced would stumble because of Him (Mt 26:31), proclaimed he’d never fall away from Jesus (Mt 26:33, 35). Yet when Jesus was arrested, Peter ran away (Mk 14:50) and later that night even denied knowing Jesus (Mt 26:69-75).
Some 15 years later, Paul confronted Peter of his falling from grace and returning to the Mosiac law out of fear (Gal 2:11-21; 5:4).
This is a man who knew his failures and what it was to be held in God’s everlasting arms (Deut 33:27), kept by the power of God (1 Pet 1:5), restored (Jn 21:15-22), and receiving an abundant entrance into the everlasting kingdom of Jesus (2 Pet 1:11).
In the third of four commands, we’re told to beware.

I. Beware (2 Pet 3:17). Pete emphasized remembering what his readers already knew (2 Pet 1:12-15; 3:1). Being forewarned in this epistle of spiritual sloth in sanctification and the dangerous character and conduct of false teachers - beware (phulasso)! This military term means to be actively, constantly and deliberately guarding (1 Cor 10:1-13). Peter used it of God guarding Noah in the Flood (2 Pet 2:5) and Paul about wolves in the assembly (Acts 20:28-30).
No one who believes Scripture can claim ignorance of spiritual things. To be forewarned is to be forearmed; but to be forewarned is also a grave responsibility, for he who knows the right and does the wrong is under a double condemnation (William Barclay, Daily Bible Study). Knowing the truth requires being on guard or we’ll become forgetful (2 Pet 1:12-15), spiritually lazy (Heb 5:11), and proud (1 Cor 10:12). Doctrinal error always results in moral failure and falling from the solid foundation of truth in Scripture (Gal 2:11-21; 5:1-6).

II. Beware of Falling (2 Pet 3:17). A Christian can fall (ekpipto) - to move from a desired position - not from salvation, but your own steadfastness (sterizo). Sterizo means to establish in a firm and secure position, to strengthen, to be immoveable. Peter’s epistles remind us that in Christ we are established (sterizo) in the present truth (2 Pet 1:12). Spiritual stability comes from knowing, believing, obeying, and using the Word to guide our thinking and living. Beware of falling from truth!
Jesus warned Peter he’d fall in sin, but prayed that Peter’s faith fail not. When restored (he didn’t lose his salvation), Peter was to spiritually strengthen his brethren (Lk 22:32). That’s what Peter did the rest of his life and does today in this epistle!
Paul warned of falling into temptation and from grace into the self-righteousness of the law (1 Cor 10:12; Gal 5:4). Ekpipto isn’t losing one’s salvation but moving downward, typically rapidly and freely without control, from a higher to a lower place. Falling is toppling from the privileges, blessings, rewards, fellowship, truth, and freedom in Christ into sin, disobedience, and chastisement. A genuine saint may fall, but Jesus and the Father keep you eternally secure (Jn 10:27-30; 1 Pet 1:3-9).

III. Beware of Being Led Astray (2 Pet 3:17). Being led away (sunapachthentes) from our own steadfastness in Christ is frightening! Led away describes being seduced; carried away as by a flood and surrendering to the current. Peter used this word twice of false teachers and their followers (2 Pet 2:2, 15).
There is grave danger in being led away with the error (plane, wandering about) of the wicked (athesimos). The word wicked describes someone who refuses to comply with the law, rebellious against authority. Like the Sodomites (2 Pet 2:7) and Balaam (2 Pet 2:18), the wicked promise you liberty, but it’s actually lawlessness against God (2 Pet 2:19). They reject what God says and live by their own desires and thoughts without a sense of responsibility to God or honor of God. Relationships with the wicked puts you in grave danger of being swept away into a spiritually destructive life. Rather, be steadfast in your faith, and let no one move you from the Word of God (Ps 1:1-6).