Verse 1 – Note that Peter starts by establishing his office as as Apostle and therefore his authority. We live in a time where we hate authority, especially in the church, so we should take special note of these things.
Verse 4 – Peter declares we have already escaped corruption and our desires for things we should not have. Since God has freed us from slavery to sin we should not go back to that bondage.
Verse 10 – We should endeavor to be sure we are actually in the faith. This means we should fully lean into Jesus’ sacrifice and not our own works. Nothing could be worse than being confident in our righteousness and then have Jesus say “depart from me, I never knew you.” (Matthew 7:21-23)
Verse 12 – Even when we know the truth we require teachers to remind us of God’s word so that we are strengthened and encouraged.
Verse 15 – Peter knows he is going to die, so he wants the “sheep” to be strong enough to go on after he is dead. The goal of a father is to make himself unnecessary. The same is true of pastors.
Verse 17 – Peter is looking back to Jesus’ baptism and the start of His ministry. There’s no need to mention what came after, because God’s approval of His Son was evidence enough. Note that even Jesus submitted to John the Baptist to be put under the water.
Verse 21 – Prophecy (and all of Scripture) is directed by the Holy Spirit. These are the words of God, not men. This is why we should not dismiss all parts of Scripture other than what is in the red letters.