I here reproduce the wise words of the Westminster Directory for the Public Worship of God on this subject, as found here.
Preaching of the Word, being the power of God unto Salvation, and one of the greatest and most excellent Works belonging to the Ministry of the gospel, should be so performed, that the Workman need not to be ashamed, but may save himself, and those that hear him.
It is presupposed…that the Minister of Christ is in some good measure gifted for so weighty a service, by his skill in the Original Languages, and in such Arts and Sciences as are handmaids unto Divinity, by his knowledge in the whole Body of Theology, but most of all in the holy Scriptures, having his senses and heart exercised in them above the common sort of Believers; And by the illumination of Gods Spirit, and other gifts of edification which (together with reading and studying of the Word) he ought still to seek by Prayer, and an humble heart, resolving to admit and receive any truth not yet attained, when ever God shall make it known unto him.…