Sin is not good. We know this. Our culture has largely lost sight of what constitutes a sin, and we may not know exactly how bad some sins are. But we can reasonably assume that if we are internally attempting to gauge the “wrongness” of an action, it is probably not the right option. We know for sure to avoid the big ones. Murder, adultery, rape and theft are all right out.

Other ones are a bit trickier. When does talking with a friend become a gossiping session? How many episodes of a TV show count as sloth? At what point does working to stay fit and healthy, and appreciating the aesthetic results, slip into vanity?

Many of these questions rely on context and individual circumstances — I imagine the moral impact of watching five episodes of The Great British Baking Show is mitigated when you are bedridden from illness or injury and not willfully ignoring other duties — but all of them require continual prayer and situational awareness lest we slip into bad habits.

St. Teresa of Ávila, a giantess among Catholicism’s holy women, knew all about the bad habits to which we weak fallen creatures are susceptible.

A woman of paradox, Teresa was a contemplative mystic and a pragmatic reformer in the 16th century, and her spiritual writings continue to bring new converts to the faith centuries after her death. Above all, she was untiring in her yearning to be with God, and to bring as many souls with her as she could. Canonized in 1622, the beloved Carmelite nun was named the first female Doctor of the Church — alongside St. Catherine of Siena — in 1970. But this firebrand, whose love for Christ echoes through eternity, almost wasted her life in habits of petty sin and vice.

As a young woman, Teresa entered the Carmelite convent in Ávila. This decision, which would bear tremendous fruit years later, at first nearly destroyed her faith. The convent was known for its leniency and worldliness. Some of the nuns had servants, and many were allowed to continue friendships with those outside the convent, hosting visitors and entertaining guests. A naturally vivacious and outgoing young woman, Teresa fell prey to the vanities and comforts around her. She lost much of the spiritual fervor she’d had as a child, when as a young girl of 7 she had attempted to run away with her brother to seek martyrdom fighting the Moors. She had chosen to take the veil in defiance of her father’s wishes, but after two decades amongst the Carmelites in Ávila, her faith had become diluted and weak.

At the age of 39, following a profound conversion after meditating upon the battered form of Christ on the Cross, Teresa forsake her acquired tepidity, and regained the zeal which had marked her early years. From this experience she determined to maintain a constant vigilance against sin in all its forms.

St Teresa had hardly been wandering around her convent committing acts of murder, adultery or theft, but upon reflecting on her first 20 years as a nun, she realized the level of danger in which she had put her immortal soul. For much of that time, she had been unaware of the cost of her small sins, hardly recognizing them as sins at all, and certainly not dissuaded from them by any spiritual authority in her life. Writing of these years, she said:

Half-instructed confessors have done my soul great harm. … They certainly did not wish to deceive me, but the fact was that they knew no better. Of something which was a venial sin, they said it was no sin, and out of a very grave mortal sin they made a venial sin. This has done me such harm, that my speaking here of so great an evil, as a warning to others, will be readily understood.

St Teresa spoke and wrote frequently on the dangers of mortal sins, but she never lost an opportunity to warn against venial sins as well, recognizing that these smaller, insidious acts wear down the soul’s defenses and ability to ward off the larger separations from God caused by grave sins. Critically, she knew of mankind’s ability to justify and excuse these actions because she had done so herself.

The late Catholic philosopher Alice Von Hildebrand once noted:

I notice in my own life as soon as the supernatural is making demands upon me which I do not like, my ‘unholy trickiness’ looks for ways of escaping.

How well St. Teresa knew this “unholy trickiness” and how it had slowly poisoned her during her first two decades as a Carmelite, and how well we all recognize it when we see it so plainly written. How often do we silently beg off our daily prayer, claiming exhaustion or distraction and promising to pray twice as much the next day? How frequently do we play up our own illnesses to justify to ourselves why we were unable to attend Mass? Do we ever forgo saying grace in public for fear of making others — but really ourselves — uncomfortable?

St Teresa knew all of these ethical gymnastics we play. She had practiced them herself for decades. By the grace of God she came face to face with Christ’s passion and saw such contortions for what they were, foolish sinfulness. What worldly comforts or applause could be worth separating herself from the love of her Creator, who gave his life for her? May we pray always for her intercession that we may be as moved by the image of our God, broken on a cross for our sins, and seek to love him better.

St. Teresa of Ávila, pray for us!