Here’s a quote from Charles Bridges’ book The Christian Ministry that addresses both the external and internal call to ministry. Bridges writes,

The external call is a commission received from and recognized by the Church…not indeed qualifying the Minister, but accrediting him, whom God had internally and suitably qualified. This call communicates therefore only official authority. The internal call is the voice and power of the Holy Ghost, directing the will and the judgment, and conveying personal qualifications. Both calls, however — though essentially distinct in their character and source — are indispensable for the exercise of our commission.

For a free PDF copy of Charles Bridges book go here.