In plain words
The four Vedas themselves took on these four forms and brought Śānti to the world. So these are not mere forms of mantra; they are truly forms with a body! Regarding those very shapes as forms of God, they developed mantra and tantra in that Tretā age. But as the ages changed, man's conduct went on changing. For the man of today there is only the form of the mantra; the embodied principle does not come along with it.
Each mantra has its own shape. Each mantra has its own inner meaning. Only when you recognise this principle and act on it do you gain the strength needed to drive the demonic away.
In the Kali age the remembrance of the Name alone is the way across
“Vedāham etaṃ puruṣaṃ mahāntam ādityavarṇaṃ tamasaḥ parastāt”: our ancient ṛṣis proclaimed that they had beheld the Paramātma, supremely radiant, who is beyond darkness. In the Dvāpara age, yogīs pictured that Divinity in their hearts and performed yajñas, yāgas and other rites. Worshipping pictures of the deities, they attained Śānti. Where there is a mantra, there is a form. Right there is the tantra too. Worshipping God through such mantra, yantra and tantra, they enjoyed Śānti. But in this Kali age, because man has completely forgotten Divinity, it has turned into an age of strife. Even between children born of the same mother, quarrels and hatreds are springing up. In such an age of strife man lacks enough strength and ability to realise Divinity, and so the elders prescribed the remembrance of the name of God. In this Kali age the remembrance of the Name alone is the way across. For mankind to cross over, a particular sādhana was laid down for each age. In the Kṛta age dhyāna, in the Tretā yajña, in the Dvāpara ritual worship, in the Kali the remembrance of the Name: so they said.