So, given the definition of knowledge as a justified true belief, ca we say we have knowledge Jesus will save us when we only have faith Jesus will save us?
Put differently, is faith sufficient to justify a true belief?
I would say that the 'faith' that Jesus will save is distinct from having 'knowledge' (in the justified, quantifiable, factual, measurable sense) that he will actually save us.
For a start, let's clarify precisely what one means by "faith": traditional Christian theology rejects the caricature that faith is simple, unthinking confidence to the truth of some postulate. That's actually a heresy we call
fideism.
The medieval scholastic theologian St. Thomas Aquinas held that faith was not mere convinced, untested opinion: on the contrary, he argued that it should be understood as a mean (in the Platonic sense) between science (i.e. demonstration) and opinion.
Certainly, the Bible and mainstream Christian tradition have never held that faith should just be accepted because it's so clear that it's the truth:
“
Test everything that is said to be sure it is true, and if it is, then accept it.” (
1 Thessalonians 5:21)
The traditional teaching has never viewed faith simply as a matter of just believing a collection of bits of information that God has revealed. St Anselm’s phrase, "
faith seeking understanding" (
fides quaerens intellectum), is applicable here.
However, the tradition also distinguishes two different species of 'knowledge'. And in this respect, the traditional Christian response (as outlined by Pope Benedict XVI in his
Introduction to Christianity) rejects the idea that factual knowledge is the only species of human knowing. Divine faith and knowledge, in Benedict's estimation, “
are two basic forms of human attitude or reaction to reality, neither of which can be traced back to the other because they operate on completely different planes.”
See:
For Benedict belief is not a lesser or incomplete form of knowledge, as in, “I believe that it may rain, but I do not know for sure.” Rather, “belief is ordered, not to the realm of what can be or has been made, although it is concerned with both, but to the realm of basic decisions that man cannot avoid making.” Belief, then, is not essentially about content and facts (though there are surely reasons for believing), but about meaning, “without which the totality of man would remain homeless, on which man’s calculations and actions are based, and without which in the last resort he could not calculate and act, because he can only do this in the context of a meaning that bears him up.”
To say, “I believe,” therefore, is not a casual remark about the weather, nor is it a sheepish disavowal of morality, as politicians and presidential candidates tend to use the phrase today. To say, “I believe,” according to Ratzinger, is to take a stand on a ground within reality which one trusts completely. This chosen ground for belief is the source of meaning in one’s life, and it cannot be verified by empirical data because it exists outside the realm of the quantifiable. Yet this ground, since it has meaning, has truth, and only in standing on truth can one understand belief, and with it, meaning...
For the Christian, taking a stand on the truth of being itself means entrusting oneself to the Logos, to the God who is meaning and reason, and therefore creative love. The Christian, according to Ratzinger, does not entrust himself to a “something” but to a “Someone” who, because of love, came into the world to make eternal meaning, truth, and love visible to all. The Christian therefore confesses, “I believe in you, Jesus of Nazareth, as the meaning (Logos) of the world and of my life.”
See this encyclical by Pope John Paul II, which cites the First Vatican Council from the 19th century:
Fides et Ratio (14 September 1998) | John Paul II
On the basis of mistaken and very widespread assertions, the rationalist critique of the time attacked faith and denied the possibility of any knowledge which was not the fruit of reason's natural capacities. This obliged the Council to reaffirm emphatically that there exists a knowledge which is peculiar to faith, surpassing the knowledge proper to human reason, which nevertheless by its nature can discover the Creator. This knowledge expresses a truth based upon the very fact of God who reveals himself, a truth which is most certain, since God neither deceives nor wishes to deceive.6
9. The First Vatican Council teaches, then, that the truth attained by philosophy and the truth of Revelation are neither identical nor mutually exclusive:
“There exists a twofold order of knowledge, distinct not only as regards their source, but also as regards their object. With regard to the source, because we know in one by natural reason, in the other by divine faith. With regard to the object, because besides those things which natural reason can attain, there are proposed for our belief mysteries hidden in God which, unless they are divinely revealed, cannot be known”.7 Based upon God's testimony and enjoying the supernatural assistance of grace, faith is of an order other than philosophical knowledge which depends upon sense perception and experience and which advances by the light of the intellect alone. Philosophy and the sciences function within the order of natural reason; while faith, enlightened and guided by the Spirit, recognizes in the message of salvation the “fullness of grace and truth” (cf. Jn 1:14) which God has willed to reveal in history and definitively through his Son, Jesus Christ (cf. 1 Jn 5:9; Jn 5:31-32)