A definition of preaching
Preaching is the proclamation of the word of God to men by men under assignment from God. It is the ordained means for the transmission of the word of God to a lost world; it serves also as an official means of grace for the building up and strengthening of the church of Christ.
Preaching is truth through personality; it may be largely didactic (to teach), or else to lead to faith (to convict and convince) and to motivate (leading to obedience and godly action). Preaching is truth on fire, radically different from a secular lecture or talk; through it the power of God is revealed unto the salvation of souls.
Through preaching men are fed with godly doctrine that leads to holiness; they are exhorted to seek God and his saving grace through Jesus Christ and His cross; they are rebuked because of error and sin; they are encouraged by the faithfulness of God presented to them in the covenant of grace.
Preaching is essential, a sine qua non for the church; without it the people of God will languish. Where true biblical, systematic and expository preaching is present the church will thrive.