Cardinal Manning states that the second effect of venial sin is to hinder the reception of grace.
We continually receive sufficient grace from God. “There is a perpetual flood, and inundation of the grace of God, coming down upon the whole race of mankind; but most especially upon those who are in the light of His faith, and in the unity of His fold. . . . The hands of Almighty God, which are infinite, are perpetually pouring out grace upon us. It is like the rain that comes down upon the sand of the shore, or upon a hungry sea, or upon the stony mountains.”
“There are two kinds of grace we are continually receiving: the one is the grace in the Sacraments; the other, the grace out of the Sacraments. The grace in the Sacraments is of two kinds. Every Sacrament has what is called the grace of the Sacrament, and also the sacramental grace.” For example: “In baptism the grace of the Sacrament is the gift of the indwelling of the Holy Ghost in the soul, by which we are made children of God: ‘the grace of adoption, whereby we can cry Abba (Father)’ (Rom 8:15). The sacramental grace is the grace of the Holy Spirit accompanying that chief grace, whereby we are enabled to fulfil all the duties that belong to the children or to the sons of God. This is the meaning of St. John, when he says, ‘To as many as received Him, to them gave He power to be made the sons of God’ (Jn 1:12). That is, every baptised person has grace from the time of his baptism to fulfil every duty of the love of God and of his neighbour, every duty of piety towards God, every duty of obedience.”
“Next, there are the graces out of the Sacrament. There are lights by which God makes the soul to know His truth, and by which He draws the soul to His presence. . . . If we would open our intellect with sincerity to receive the light of truth, and our conscience to receive the attraction of Divine grace, it would fill and illuminate us; but by faults of self-indulgence, worldliness, fear of man, and human respect, we bring a film over our eyes, and the inward eye of the intellect and conscience at last loses its faculty of discernment.” Accordingly, a person’s venial sins hinder his reception of grace.
Quotations from Henry Edward Manning, Sin and Its Consequences, 2d ed. (London: Burns and Oates, 1874).