The tense Peter uses of the verb I have written
communicates the thought that his letter is complete. The adverb briefly
admits the fact that his letter is succinct, and so potentially open for the need for further clarity. Silvanus, of course, is the man who could give an initial expansion.
12 By Silvanus, a faithful brother as I regard him, I have written briefly to you, exhorting and declaring that this is the true grace of God. Stand firm in it.