What are the practical effects of sanctifying grace?

Sanctifying grace is a habitual gift, a stable and supernatural disposition that perfects the soul itself to enable it to live with God, to act by his love. Habitual grace, the permanent disposition to live and act in keeping with God's call, is distinguished from actual graces which refer to God's interventions, whether at the beginning of conversion or in the course of the work of sanctification.

CCC - 2000

This quote sums it up pretty nicely.

Everything is built on sanctifying grace. We have no life in us. John 6:53. You can read so much about the Eucharist and its effects on our lives. We wouldn't have saints if there are no practical effects of sanctifying grace. The ability to stay in a state of grace and not fall into mortal sin is thanks to the sanctifying grace.

To the question of "Whether man can know that he has grace?" (Summa Theologica I-II q. 112 a. 5), St. Thomas Aquinas responds by explaining the ways the reality of grace can or cannot be known:

There are three ways of knowing a thing:

1. by revelation, and thus anyone may know that he has grace, for God by a special privilege reveals this at times to some, in order that the joy of safety may begin in them even in this life, and that they may carry on toilsome works with greater trust and greater energy, and may bear the evils of this present life, as when it was said to Paul (2 Cor. 12:9): "My grace is sufficient for thee."

2. a man may, of himself, know something, and with certainty; and in this way no one can know that he has grace. For certitude about a thing can only be had when we may judge of it by its proper principle. Thus it is by undemonstrable universal principles that certitude is obtained concerning demonstrative conclusions. Now no one can know he has the knowledge of a conclusion if he does not know its principle. But the principle of grace and its object is God, Who by reason of His very excellence is unknown to us, according to Job 36:26: "Behold God is great, exceeding our knowledge." And hence His presence in us and His absence cannot be known with certainty, according to Job 9:11: "If He come to me, I shall not see Him; if He depart I shall not understand." And hence man cannot judge with certainty that he has grace, according to 1 Cor. 4:3,4: "But neither do I judge my own self … but He that judgeth me is the Lord."

3. things are known conjecturally by signs; and thus anyone may know he has grace, when he is conscious of delighting in God, and of despising worldly things, and inasmuch as a man is not conscious of any mortal sin. And thus it is written (Apoc. 2:17): "To him that overcometh I will give the hidden manna … which no man knoweth, but he that receiveth it," because whoever receives it knows, by experiencing a certain sweetness, which he who does not receive it, does not experience. Yet this knowledge is imperfect; hence the Apostle says (1 Cor. 4:4): "I am not conscious to myself of anything, yet am I not hereby justified," since, according to Ps. 18:13: "Who can understand sins? From my secret ones cleanse me, O Lord, and from those of others spare Thy servant."

How refreshingly honest of your daughter to say what she thinks rather than simply give the answer that is expected. But how prove to her that sanctifying grace is real? By example, by demonstrating to her that the practical effects of sanctifying grace result in a transformed life to; to illustrate how God’s sanctifying grace sets believers apart for holiness.

This is my understanding of the Catholic view of sanctifying grace:

Sanctifying grace refers to a specific supernatural infusion of God’s grace that makes a person holy and pleasing to God. Deifying grace and perfecting grace are other terms for sanctifying grace, which is believed to be imparted through the Catholic sacrament of baptism. The Roman Church teaches that at baptism, the time when sanctifying grace is received, a person becomes part of the body of Christ and able to receive additional graces for living a holy life.

According to the Catechism of the Catholic Church, sanctifying grace is also called the “grace of justification” because it is the grace that makes a soul acceptable or justified before God. This grace transforms a sinner into a holy child of God. Source: What is sanctifying grace?

I realise this comes from a Protestant source, so if this is inaccurate or misleading, please say so.

Another approach might be to simply explain to your daughter that if she wishes to experience the free and undeserved gift of God’s sanctifying grace, first she has to acknowledge it is real, second that she needs it more than oxygen, and third that she first has to empty her hands in order to receive it.

For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God (Ephesians 2:8).

For what it’s worth, when I first came to saving faith in Christ Jesus and presented myself for adult baptism with my husband and our son present, our son asked me, “Mum, does this mean you have to become holy?” Our son was only 12 years old. He intuitively realised that my public declaration of my Christian faith necessitated a big change in my life! Out of the mouths of babes and children...

Perhaps this is the only evidence that matters to children – that they can see how Christians live a life that is different to others, that God’s sanctifying grace has transformed their lives. It’s a case of walking the walk, rather than talking the talk. Children are not easily fooled! Role models might be the answer here - real-life, in your face, modern and even public figures who exemplify the ongoing process of sanctification and grace in their lives, who are a witness to the world that they have been separated from the world and set apart to be holy.

The proof of Sanctifiying Grace is found in the testimony of God’s people. Can you talk about the changes in your own life that have come only because of your realtionship with God? The initial changes at the time of Salvation can be powerful, there are also great stories of another process of sanctification in those who seek God. Find testimonies that can provide insight into the whole concept.