Guptasadhana TantraA mantra is, like everything else, Shakti. But the mere utterance of a mantra without more is a mere movement of the lips.
This tantra is likely quite old. It is quoted as a source in the Matrikabhedatantra, which may possibly date from the 13th century c.e. The edition used for this abstract is No. 311 in the Chowkhamba Haridas Sanskrit series. The work is unabashedly of a Kaula slant, briefly exposed in twelve short patalas (chapters).