The American Church Dictionary and Cycopedia
The words, "In the Name of the Father, and of the Son, and of the Holy Ghost," used before sermons, is so called; to which the people respond "Amen." This is a very ancient usage, and founded on the belief that so important a work as "preaching the Word" should be done in the Name of the Lord. The Invocation is the name given also to the third paragraph of the Prayer of Consecration in the Communion Office, in which the Merciful Father is invoked that He may "vouchsafe to bless and sanctify with Thy Word and Holy Spirit, these Thy gifts and creatures of bread and wine, that we, receiving them according to Thy Son our Saviour Jesus Christ's holy institution, in remembrance of His Death and Passion, may be partakers of His most blessed Body and Blood."
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Miller, William James. Entry for 'Invocation, the'. The American Church Dictionary and Cyclopedia. https://www.studylight.org/dictionaries/eng/acd/i/invocation-the.html. 1901.