This text from Prof. Plinio Correa de Oliveira explains what is this major point of the soul, or primordial light:
According to Catholic theology, for the same reason that each one of us feels within himself a tendency towards a specific sin—usually referred to as capital sin—in he contrary sense, each soul is called to reflect a specific aspect of God by especially shining in the practice of a specific virtue. This has been referred to as the person's "primordial light."
Thus we may conjecture that as one advances in sanctity, his primordial light becomes more evident. Were we to correspond faithfully to every grace that Christ gives, His light would radiate through our poor selves.
One person tends to be charitable, another loyal, another obedient, and yet another serious and responsible. One is inspired by all that is pure and sublime, another by the severe and austere.
One has such a love for our Blessed Mother that he cannot hear her name mentioned without feeling every string of his heart stir. Another is particularly touched by all that surrounds the birth of the Child Jesus, yet another by Our Lord's Passion. All souls are called to practice all virtues, but a particular virtue shines before each in a primordial, a first light.
Imagine Saint Louis Gonzaga, the personification of purity, and emulate his angelic chastity. Consider Saint Louis, King of France, the embodiment of honor, with uprightness and sincerity written in each line of his noble face.
Reflect on Saint Vincent de Paul, an emissary of divine charity, who walked the back streets of Paris rescuing abandoned babies and carrying them in the huge pockets of his cassock to their new homes. Recall Saint Francis of Assisi, who courted Lady Poverty throughout his life, or Saint John the Baptist, who embodied the rigors of God's call to repentance and penance.
As every virtue reflects its divine Author, Saint Thomas concludes that Christ is the perfect expression of all the primordial lights that were, are, and will be. To that we may add that every saint is nothing more–nor less–than a small spark of the perfection of Our Lord, an inestimable honor indeed.