"Faith in God, sustained by divine grace, is therefore a very important element for living in mutual dedication and conjugal fidelity," he said.
The Pope said he was not suggesting there was a simple, automatic link "between the lack of faith and the invalidity of marriage."
Rather, he hoped "to draw attention to how such a lack may, although not necessarily, also hurt the goods of marriage," given that referring to God's plan "is inherent in the covenant of marriage."
The Pope made his comments Jan. 26 during a meeting with members of the Roman Rota, a Vatican-based tribunal that deals mainly with marriage cases.
The current crisis of faith has brought with it a state of crisis for the Christian vision of marriage as an indissoluble bond between a man and a woman, the Pope said.
"The indissoluble covenant between man and woman does not require, for the purpose of sacramentality, the personal faith of those to be married," he said. "What is required, as the minimum condition, is the intention of doing what the church does" when it declares a marriage is a sacrament.
While the question of intent should not be confused with the question of the individuals' personal faith, "it is not always possible to completely separate them," he said.
The Pope quoted Blessed John Paul II's speech to the Vatican court in 2003 in which he said, "an attitude on the part of those getting married that does not take into account the supernatural dimension of marriage can render it null and void only if it undermines its validity on the natural level on which the sacramental sign itself takes place."
"The Catholic Church has always recognized marriages between the non-baptized that become a Christian sacrament through the baptism of the spouses," and it does not doubt "the validity of the marriage of a Catholic with a non-baptized person if it is celebrated with the necessary dispensation," the late Pope had said.
Pope Benedict said such considerations need further reflection, especially in a secularized culture that puts little faith in a person's ability to make a lifelong commitment and fosters an incorrect understanding of freedom and fulfillment.
Humanity is incapable of achieving what is truly good without God, the Pope said, and refusing God's invitation "leads to a deep imbalance in all human relationships," including marriage.
While faith in God is "a very important" part of a marriage lived with commitment and loyalty, it does not mean that "loyalty and other (conjugal) properties are not possible in natural marriage between non-baptized" spouses, who still receive the graces that come from God.
"However, closing oneself off from God or refusing the sacred dimension of the conjugal union and its value in the order of grace certainly makes it more difficult to realize concretely the highest model of marriage as envisioned by the church according to God's plan, possibly going so far as to undermine the actual validity of the covenant" if the tribunal determines it amounts to a refusal of fidelity, procreativity, exclusivity and permanence.
Faith, therefore, "is important in the realization of the authentic conjugal good, which consists simply in always wanting the good for the other, no matter what," together with a true and indissoluble partnership for life, he said.
Faith without charity, which is love, "bears no fruit, while charity without faith would be a sentiment constantly at the mercy of doubt. Faith and charity each require the other, in such a way that each allows the other to set out along its respective path," the Pope said, citing his 2011 apostolic letter "Porta Fidei" ("The Door of Faith").
In addition to the three goods of procreation, marital fidelity and its indissolubility, "one must not exclude the possibility of cases in which, exactly because of the absence of faith, the good of the spouses ends up compromised and, therefore, there is a lack of consent," the Pope said.
It's not the first time Pope Benedict has called for a closer reflection on the impact of an absence of faith in determining marriage annulments.
During an unscripted question-and-answer session with priests in northern Italy in 2005, the Pope noted the problem of people who married in the church not because they were believers but because they wanted a traditional ceremony.
He said that when he was prefect of the Congregation for the Doctrine of the Faith, he asked several bishops' conferences and experts to study the problem, which in effect was "a sacrament celebrated without faith."
He said he had thought that the church marriage could be considered invalid because the faith of the couple celebrating the sacrament was lacking. "But from the discussions we had, I understood that the problem was very difficult" and that further study was necessary, he said.