"gooseberry in the hand", for example. Certainly, it's not about practical application of ordinary mustard grains. It is a metaphor. Though, in ancient India the mustard seed was considered as a magical substance that could help counteract all hindrances and inauspicious turns of fate, is also used for the ritual expulsion of spirits and therefore, a wrathful means of overcoming obstacles. Mustard seed is one of the sacrificial ingredients offered to Agni. In Buddhism, the wrathful form of Vajrapani, the Bodhisattva of power, was said to have offered mustard seed to the Buddha. It is more important that the mustard seed has been widely used not just as a symbol of the smallest point, but of that of which the largest emerges. It's about the hidden but so hugest potential. It refers to the Faith. To the content that is potential capacity within all human beings, even the agnostic.
"Atman [Self], residing in the lotus of the heart--is smaller than a grain of paddy, than a barley corn, than a mustard seed, than a grain of millet or than the kernel of a grain of millet. This, my Atman residing in the lotus of the heart is greater than the earth, greater than the sky, greater than heaven, greater than all these worlds." (Chandogya Upanishad)
"The one I call holy does not cling to pleasures, like water on a lotus leaf or a mustard seed on the point of a needle." (Dhammapada)
He told them another parable: "The kingdom of heaven is like a mustard seed, which a man took and planted in his field. Though it is the smallest of all seeds, yet when it grows, it is the largest of garden plants and becomes a tree, so that the birds come and perch in its branches." (Matthew 13:31; Mark 4:30; and Luke 13:19)
"The gate of liberation is narrow, less than one-tenth of a mustard seed. The mind has become as big as an elephant; how can it pass through this gate? If one meets such a True Guru, by His Pleasure, He shows His Mercy. Then, the gate of liberation becomes wide open, and the soul easily passes through." (Guru Granth Sahib)
The seed in general is a deep symbol in many religions. And the Hindu tilaka is also the recognition of that seed of divinity in all beings.
Despite our imperfection, weaknesses, and faults, our seemingly small beginnings of spiritual awakening, like the seed, are capable of immense growth. But while there is tremendous potential, of itself it does not do the growing. It depends on His Grace
and our ability to trust Him with the seed of ourselves.