Fr. John Hardon, S.J. summarises the Church’s infallible doctrine on capital punishment:

“Over the centuries, the Church’s writers have defended the traditional Christian teaching on capital punishment.

St. Augustine explained St. Paul’s teaching on the State’s right to inflict capital punishment. Certainly the State may execute convicted criminals. But it should exercise Christian forbearance and thus temper juridical severity.

Pope Leo I in the fifth century and Pope Nicholas I in the ninth century made it clear that the Church herself could not be directly involved in capital punishment; but the pontiffs assumed that the State was divinely authorized to do so. So, too, the Councils of Toledo (675) and Fourth Lateran (1215) forbade the clergy to take direct part in the juridical process or sentencing of a person on a capital charge. But again, the councils took for granted that the State may condemn a convicted criminal to death and execute the sentence.

St. Thomas Aquinas made the classic defense of capital punishment. He reasoned that “if a man be dangerous and infectious to the community, on account of some sin, it is praiseworthy and advantageous that he be killed in order to safeguard the common good” (Summa Theologica II, II, 64, 2). Certainly the crime had to be very serious, and the welfare of society was at stake. But there was no question about the moral validity of capital punishment.

In the sixteenth century, the Council of Trent mandated the publication of the Roman Catechism. Promulgated by Pope St. Pius V in 1566, it has been confirmed by one sovereign pontiff after another. Thus in 1905, when Pope St. Pius X decreed the catechetical instruction to be given in the Catholic world, he mandated that the basis of this instruction should be the Roman Catechism. In dealing with the fifth commandment of the Decalogue, this fundamental catechism of Catholic doctrine declares:

“There are some exceptions to the extent of this prohibition to killing. The power of life and death is permitted to certain civil magistrates because theirs is the responsibility under law to punish the guilty and protect the innocent. Far from being guilty of breaking this commandment, such an execution of justice is precisely an act of obedience to it. For the purpose of the law is to protect and foster human life. This purpose is fulfilled when the legitimate authority of the State is exercised by taking the guilty lives of those who have taken innocent life” (Roman Catechism, The Fifth Commandment, 4).

In the twentieth century, Pope Pius XII provided a full doctrinal defence of capital punishment. Speaking to Catholic jurists, he explained what the Church teaches about the authority of the State to punish crimes, even with the death penalty.

The Church holds that there are two reasons for inflicting punishment, namely “medicinal” and “vindictive.” The medicinal purpose is to prevent the criminal from repeating his crime, and to protect society from his criminal behaviour. The vindictive is to expiate for the wrong-doing perpetrated by the criminal. Thus reparation is made to an offended God, and the disorder caused by the crime is expiated.

Equally important is the pope’s insistence that capital punishment is morally defensible in every age and culture of Christianity. Why? Because the Church’s teaching on “the coercive power of legitimate human authority” is based on “the sources of revelation and traditional doctrine.” It is wrong, therefore “to say that these sources only contain ideas which are conditioned by historical circumstances.” On the contrary, they have “a general and abiding validity.” (Acta Apostolicae Sedis, 1955, pp 81-2).

Behind this declaration of the Vicar of Christ is a principle of our Catholic Faith. Most of the Church’s teaching, especially in the moral order, is infallible doctrine because it belongs to what we call her ordinary universal magisterium. There are certain moral norms that have always and everywhere been held by the successors of the Apostles in communion with the Bishop of Rome. Although never formally defined, they are irreversibly binding on the followers of Christ until the end of the world.

Such moral truths are the grave sinfulness of contraception and direct abortion. Such, too, is the Catholic doctrine which defends the imposition of the death penalty.

Certainly Christianity, like Christ, is to be merciful. Certainly Christians are to be kind and forgiving. But Christ is God. He is, indeed loving and in fact is love. But He is also just. As a just God, He has a right to authorise civil authority to inflict capital punishment.”