The answer comes in his essay: “Thoughts Concerning Gospel Ministers”:
Who is a Gospel Minister, in the full, scriptural sense of the word? He, and he alone, of whatever denomination, that does declare the whole counsel of God; that does preach the whole gospel, even justification and sanctification, preparatory to glory. He that does not put asunder what God has joined, but publishes alike, “Christ dying for us, and Christ living in us”. He that constantly applies all this to the hearts of the hearers, being willing to spend and be spent for them; having himself the mind which was in Christ, and steadily walking as Christ also walked; he, and he alone, can with propriety be termed a Gospel Minister.
Let it be particularly observed, if the gospel be “glad tidings of great salvation which shall be to all people”, then those are, in the full sense, Gospel Ministers who proclaim the “great salvation”; that is, salvation from all (both inward and outward) sin, into ” all the mind that was in Christ Jesus”; and likewise proclaim offers of this salvation to every child of man. This honorable title is therefore vilely prostituted, when it is given to any but those who testify “that God willeth all men to be saved”, and “to be perfect as their Father which is in heaven is perfect”.