The Roman Catholic Magisterium has definitive teachings on what makes an act a sin, and what makes one act a venial sin and another act a mortal sin. There are three things that can possibly make an act, the knowing choice of a human person, a sin. These three things are called the three fonts of morality:
1. intention — the end intended by the person who acts; it is the reason or purpose for choosing the act. If the intended end is gravely disordered, then the act is a mortal sin.
2. moral object — the deliberately chosen act has its own end, toward which it is intrinsically directed; this end is called the moral object. If the moral object is evil, the act is intrinsically evil. If there is only good in the moral object, the act is intrinsically good. An act can have more than one moral object; if any one moral object is evil, the act is intrinsically evil, regardless of how many other good moral objects are also in the act.
The deliberate and knowing choice of an act that is inherently ordered toward an evil moral object (an evil proximate end) is always a sin. Any act with an evil moral object is intrinsically evil and always immoral. But some intrinsically evil acts are venial sins, while other intrinsically evil acts are mortal sins. If the moral object is gravely disordered, then the act is a mortal sin.
3. circumstances — the person chooses the act with the reasonable anticipation that the act will have consequences. The reasonably anticipated good and bad consequences must be weighed, and if the bad outweighs the good, the act is a sin. If the reasonably anticipated bad consequences gravely outweigh the good consequences, then the third font is gravely disordered and the act is a mortal sin.
If any one or more fonts is gravely disordered, the act is always objectively a mortal sin. A grave moral disorder occurs whenever the knowing choice of such an act would be objectively incompatible with the love of God above all else, and the love of neighbor as self. A lesser disorder in an act makes the act a venial sin, rather than a mortal sin.
If one font of morality is gravely disordered, the act is a objectively a mortal sin, no matter how good the other two fonts of morality may be.
From my book, The Catechism of Catholic Ethics:
.021. A venial sin is an act that is not so gravely immoral before God as to be entirely incompatible with true love of God and neighbor. An actual venial sin does not include sufficient culpability to take away the state of grace from the soul, nor to deserve eternal damnation. A venial sin is always in some way contrary to true love of God and neighbor, but to a substantially limited extent. An actual venial sin always includes some culpability and some lack of cooperation with grace, and always deserves some degree of punishment.
A mortal sin is an act that is so gravely immoral before God as to be entirely incompatible with true love of God and neighbor. An actual mortal sin includes sufficient culpability to take away the state of grace from the soul, and to deserve eternal damnation.
Saint Thomas Aquinas: “Therefore when the soul is so disordered by sin as to turn away from its last end, viz. God, to Whom it is united by charity, there is mortal sin; but when it is disordered without turning away from God, there is venial sin.” (St. Thomas Aquinas, Summa Theologica, I-II, Q. 72, A. 5.)
Pope John Paul II: “And when through sin, the soul commits a disorder that reaches the point of turning away from its ultimate end, God, to which it is bound by charity, then the sin is mortal; on the other hand, whenever the disorder does not reach the point of a turning away from God, the sin is venial. For this reason venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity and therefore eternal happiness, whereas just such a deprivation is precisely the consequence of mortal sin.” (Pope John Paul II, Reconciliation and Penance, n. 17; he cites St. Thomas Aquinas, Summa Theologica, I-II, Q. 72, A. 5.)
Mortal sin differs from venial sin both by degree and by type. Any mortal sin is more serious than any venial sin, so they differ by degree. Mortal sins are greater in degree, since they offend God more. But mortal sin is also a different type of sin, the type that deserves eternal punishment. No one is ever sent to Hell merely for unrepentant venial sins. But one unrepentant actual mortal sin is sufficient to condemn the person to eternal punishment in Hell. (Pope Benedict XII, On the Beatific Vision of God; Council of Florence, 6 July 1439.)
On the topic of lying, a lie is a mortal sin if:
1. the intention is gravely immoral, such as intending to do grave harm to another human person, or intending to obtain what you want regardless of the harm to others;
2. the truth being denied is of grave moral weight, such as denying a moral or religious truth, or lying on a grave matter, such as the commission of a grave crime;
3. the reasonably anticipated harm done by the lie would gravely outweigh any good done.
Lying is intrinsically evil and always a sin. But a lie is only a venial sin, unless any one of the fonts of morality is gravely disordered. A lie is properly called a ‘white lie’ only if the lie is not in any way gravely disordered. However, all lies, even when not gravely disordered, are sinful. For no lie is of the truth. And God is Truth.