To preach simply means to proclaim. Politicians preach but that doesn't make their preaching right. Biblical preaching is only about one thing; proclaiming the gospel of Christ to the lost. That's it; nothing else. Preaching to the regenerate only keeps them in their spiritual immaturity. Preachers who preach to the saved are performing an extra-Biblical task. We are to preach to the lost & to teach believers the deeper truths of Scripture.
It would seem that 2 Timothy 3:16 - 4:3 would say otherwise.
2Ti 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
2Ti 3:17 That the man of God may be perfect, throughly furnished unto all good works.
2Ti 4:1 I charge thee therefore before God, and the Lord Jesus Christ, who shall judge the quick and the dead at his appearing and his kingdom;
2Ti 4:2 Preach the word; be instant in season, out of season; reprove, rebuke, exhort with all longsuffering and doctrine.
2Ti 4:3 For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears;