How do we know that Paul wrote only to believers?
Paul's letters consistently open by addressing 'the saints' or 'the church of God,' and Scripture defines every believer as a saint, not an elite few. Therefore the Pauline epistles are written to believers. Ephesians 1:1 and Ephesians 2:1 illustrate this pattern.
Les explains that all through Paul's epistles he keeps hammering home that Christ died for our sins, and that as He died we have to die, so out of that death we get new life, eternal life—not just for this life but forever. Then in Ephesians 2:1 Paul writes again to believers. To demonstrate this, Les turns back to Ephesians 1:1, where Paul, an apostle of Jesus Christ by the will of God, writes 'to' the saints. The word 'to' shows who he is writing to. The saints are not three or four elite people but every believer; every believer is a saint according to Scripture. So these Pauline letters all start the same way: Paul, an apostle of Jesus Christ to the saints, or to the church of God in such and such a place. That is where Les gets it that Paul writes only to the believer. Then in Ephesians 2, he writes to us as believers, saying 'and you, as a believer, he hath'—past tense, already done—and the next word is given as 'wicked,' meaning made alive.
Paul, an apostle of Jesus Christ by the will of God, to the saints which are at Ephesus, and to the faithful in Christ Jesus: