Textual Analysis (My Argument)
Contrasting Mere Believers from what Believers are Called to Become
I believe the context answers this:
v.1 The difference is established in v.1, but it is not "us apostles" and "you (who have obtained a faith of equal standing with ours)." Rather, it is setting up a contrast between "servants [δοῦλος (doúlos)]", willing indentured bond-slaves (whether apostle or not) to Christ, and simply "believers." The "us" in v.1 is a reference to these servants of faith, which these believers he is writing to are linked with. In so linking, the "us" of v.3 becomes inclusive of what has been given to all believers (all those of like faith), while the "you" becomes a reference to those that have not yet taken on the δοῦλος calling, which is the distinction that remains, and which Peter begins to address.
v.2 Peter prays that (all English translations from ESV; all bold emphasis added):
"grace and peace be multiplied to you in the knowledge of God and
of Jesus our Lord"
God desires a growth beyond mere belief, so God through Peter1 expands upon what this knowledge is that will make one not just believers, but servants of God and Christ, and as such have "grace and peace multiplied."
v.3 God calls believers to be more, to be servants, which requires
His divine power has granted to us [believers] all things that pertain to life and
godliness, through the knowledge of him who called us [believers] to his own
glory and excellence,
v.4 God is calling believers in the world (this side of future glory) to become present partakers of "the divine nature," which requires some actions to be taken.
by which he has granted to us [believers] his precious and very great promises, so
that through them you [believers not yet godly and serving] may become partakers of the divine nature,
having escaped from the corruption that is in the world because of
v.5-7 There is a progression to follow in spiritual growth toward service and ultimately toward being like the divine nature:
5 For this very reason, make every effort to supplement
your faith [start point are those with faith, the subjects of the letter] with virtue [moral, external behavior], and virtue with knowledge [of God and Christ], 6 and
knowledge with self-control [internal control], and self-control with steadfastness [consistency in the former actions], and
steadfastness with godliness [devotion to God], 7 and godliness with
brotherly affection [genuine care and concern for others], and brotherly affection with love [selfless self-giving for others benefit].
v.8-11 These qualities (from vv.5-7) are not automatic to a believer, but should be worked on and ever growing. In this way, the knowledge one has of Christ not only has born faith in them, but in turn bears faith in others; and it also affirms that they are true believers.
8 For if these qualities are yours and are increasing, they
keep you from being ineffective or unfruitful in the knowledge of
our Lord Jesus Christ. 9 For whoever lacks these qualities
is so nearsighted that he is blind, having forgotten that he was
cleansed from his former sins. 10 Therefore, brothers, be
all the more diligent to confirm your calling and election, for if you practice these qualities you will never fall.
11 For in this way there will be richly provided for
you an entrance into the eternal kingdom of our Lord and Savior Jesus
There is a greater reward, one more "richly provided," to those who move beyond mere faith to actual service of God. But it all hinges on the choice and action (the whole passage is filled with action and growth terms) to become a bond-servant to Christ, not just to have faith in what He has done, but to live out that faith to reflect the divine nature and bring others to Christ.
So the "us" is inclusive of all believers, who are granted those things noted, which things are designed to move the "you" who are merely believers to the same state as Peter and many other believers, a state of service and fruitfulness to God—but only if they choose to follow the process.
Disclaimer: These are primarily from a Protestant perspective.
"Us" Believers without clarity on the "you"
Thomas R. Schreiner, 1, 2 Peter, Jude, Vol. 37, The New American Commentary ( Nashville: Broadman & Holman Publishers, 2003), 291-296, states:
The word “us” refers to all believers, not merely the apostles or
Schreiner is not clear about the switch to "you," and what significance that may have. He collapses the "you" to be all believers also, saying:
What Peter meant by this ['you may participate in the divine nature'] is that believers are promised that they will
be like God.
That makes the "you" equal to the "us."
A similar "us" (believers) without explicit notation of the significance of "you" (also collapsing that into simply what Christians are called to without noting the fact that Peter does not consider himself as still needing to attain this) is found in (1) John F. Walvoord, Roy B. Zuck, Dallas Theological Seminary, The Bible Knowledge Commentary: An Exposition of the Scriptures (Wheaton, IL: Victor Books, 1985), 2:864-865; (2) Robert James Utley, The Gospel According to Peter: Mark and I & II Peter, Volume 2. Study Guide Commentary Series (Marshall, Texas: Bible Lessons International, 2000), 276-277; (3) D. A. Carson, R. T. France, J. A. Motyer, and G. J. Wenham, eds. New Bible Commentary: 21st Century Edition, 4th ed. (Leicester, England; Downers Grove, IL: Inter-Varsity Press, 1994), 1389-1390; (4) Robert B. Hughes and J. Carl Laney, T*yndale Concise Bible Commentary*, The Tyndale Reference Library (Wheaton, IL: Tyndale House Publishers, 2001), 698.
All of these mention concepts of believers growing, being transformed, sanctified, etc. (like what I argued above). They just do not address the switch of persons, and why Peter may not be including himself in when using the "you."
"Us" as Apostles
W. Robertson Nicoll, The Expositor’s Greek Testament: Commentary, Vol. 5 (New York: George H. Doran Company, n.d.), 124, does emphasize the switch in persons, but to a differing conclusion, and states of v.3-4:
Throughout this passage, the contrast between ἡμῖν, ἡμᾶς, and 2 p.
plur. in γένησθε (ver. 4) must be preserved. ἡμῖν implies the
apostolic circle, who, by virtue of their own experience of the δόξα
and ἀρετή of Christ, are able to transmit to these readers certain
promises “precious to us, and glorious.” (So Spitta, Van Soden).
1 I hold to a hermeneutic that views Scripture as a united whole, as much God's word as the words of the human writer, so God is revealing things through Peter.