"Preach the word! Be ready in season and out of season. Convince, rebuke, exhort, with all longsuffering and teaching." (2 Tim. 4:2)
One of the greatest impacts of the Protestant Reformation was the re-establishment of the priority of the preached Word of God in the church. The "sermon" (from a Latin term that means "a talk" or "a discourse") became the center of worship, rather than the Eucharist (the Lord's Supper), as in the Roman Mass.
The Directory for Worship of the Orthodox Presbyterian Church (chapter 3, section 3) gives a powerful statement about the sermon:
In the sermon God addresses the congregation by the mouth of his servant. It is a matter of supreme importance that the minister preach only the Word of God, not the wisdom of man, that he declare the whole counsel of God, and that he handle aright the Word of truth. To these ends the sermon must be prepared with the utmost care. Let the session give diligence that no person enter the pulpit concerning whose doctrinal soundness or knowledge of Scripture there is reasonable doubt. A text may not be used merely to introduce a sermon but must be painstakingly expounded. In the sermon the minister should explain the Word of God for the instruction of his hearers and then apply it for their exhortation. Care should be taken in preaching that Christian duty be not divorced from Christian truth. That minister fails to perform his task as a God-appointed watchman on Zion's walls who neglects to warn the congregation of prevalent soul–destroying teachings by enemies of the gospel. The minister should seek to perfect the saints by building them up in the most holy faith and in Christ's stead should beseech the unconverted that they be reconciled to God. Nothing is more necessary than that the gospel of salvation by grace be proclaimed without any adulteration or compromise, in order that the unsaved may rely for salvation on the grace of God only, to the exclusion of their own works or character, and that the saints may ascribe glory for their salvation to God alone.