“The first and principal duty of a pastor is to feed the flock by diligent preaching of the word. It is a promise relating to the new testament, that God would give unto his church “pastors according to his own heart, which should feed them with knowledge and understanding” (Jer. 3:15). This is by teaching or preaching the word, and no otherwise. This feeding is of the essence of the office of a pastor, as unto the exercise of it; so that he who doth not, or can not, or will not feed the flock is no pastor, whatever outward call or work he may have in the church. The care of preaching the gospel was committed to Peter, and in him unto all true pastors of the church, under the name of “feeding” (John 21:15-17). According to the example of the apostles, they are to free themselves from all encumbrances, that they may give themselves wholly unto the word and prayer (Acts 6:1-4). Their work is “to labour in the word and doctrine (I Tim. 5:17); and thereby to “feed the flock over which the Holy Ghost hath made them overseers” (Acts 20:28): and it is that which is everywhere given them in charge.
“This work and duty, therefore, as was said, is essential unto the office of a pastor. A man is a pastor unto them whom he feeds by pastoral teaching, and to no more; and he that doth not so feed is no pastor. Nor is it required only that he preach now and then at his leisure, but that he lay aside all other employments, though lawful, all other duties in the church, as unto such a constant attendance on them as would divert him from this work, that he give himself unto it—that he be in these things labouring to the utmost of his ability. Without this no man will be able to give a comfortable account of the pastoral office at the last day.”
(John Owen, Works, vol. 16, pp. 74-75)