Why could Peter write about salvation in 1 Peter 2 after Paul's revelations had been around for a long time?
Peter could write this because his epistle was composed shortly before both Peter and Paul were martyred. By that late date, Paul's epistles and doctrine had already been circulating for a long time, so Peter could come to this conclusion and address believing Jews of the dispersion, with application also to us.
Les explains that after the revelations of Paul had been around for a long, long time, Peter can write in 1 Peter 2. These little epistles were written just shortly before Peter and Paul were both martyred, which means Paul's epistles and Paul's doctrine had been out there a long time already. So Peter can come to this kind of conclusion. In 1 Peter 2, Peter writes to believing Jews of the dispersion, but that does not mean it does not apply to us. He uses the analogy of newborn babes desiring the milk of the Word so they may grow, and of living stones being built up a spiritual house, a holy priesthood, to offer up spiritual sacrifices by Jesus Christ.