In their book Handbook of Christian Apologetics, Peter Kreeft and Ronald K. Tacelli give a summary of faith.
Kreeft and Tacelli say we must distinguish between the act of faith from the object of faith- believing from what is believed. The object of faith means all things believed. For the Christian, this means everything God has revealed in the Bible. This faith (the object, not the act) is expressed in propositions. Propositions are many, but the ultimate object of faith is one. The ultimate object of faith is not words, but God’s Words (singular), indeed-Himself.
Without a relationship with the living God, propositions are pointless, for their point is to point beyond themselves to God. But without propositions, we cannot know or tell others what God we believe in and what we believe about God. They mention a few aspects of faith here:
1. Emotional faith: is feeling assurance or trust or confidence in a person. This includes hope (which is much stronger than a wish and peace (which is much stronger then mere calm.).
2. Intellectual faith: is belief. It is this aspect of faith that is formulated in propositions and summarized in creeds.
3. Volitional faith: is an act of the will, a commitment to obey God’s will. This faith is faithfulness, or fidelity. It manifests itself in behavior, that is, in good works.
Belief That and Belief In
In many cases, there are times in life where we may say the following:
“I believe that my fiancé is the right one for me and I am going to marry her”
“I believe that this job is the best job for me because I am qualified to do it.”
“I believe that this house is the best investment for us as a family.”
Notice that in each case I say “I believe that.”
Now let’s contrast that with “Belief In”
“Belief in” is when we take a step of trust, or commitment. Note that #3 above where it says, volitional faith is an act of the will, a commitment to obey God’s will. This faith is faithfulness, or fidelity. We exercise our will and move beyond just having propositional evidence for something. For example, in James 2:19, it says that the demons believe that God exists. But just because the demons think that God exists, this doesn’t mean they have “belief in” the one true God.
Now when we go from “belief that” to “belief in,” we are not going to have 100 percent certainty. For example, when we get married, take job, buy a home, we commit and do take a risk. We have gaps of knowledge, some level of uncertainty, or perhaps unanswered questions. Obviously, we assume everything will work out. But we still take a step of faith or trust. And we are required to make a 100 percent commitment. Yes, it can hard to maintain that commitment.
To build on this, here is a biblical definition faith:“ These terms refer to the value of reliability. The value is ascribed to persons as well as to objects and qualities. Relative to persons, faith is reliability in interpersonal relations: it thus takes on the value of enduring personal loyalty, of personal faithfulness. The nouns ‘faith’, ‘belief’, ‘fidelity’, ‘faithfulness,’ as well as the verbs ‘to have faith’ and ‘to believe,’ refers to the social glue that binds one person to another. This bond is the social, externally manifested, emotionally rooted behavior of loyalty, commitment, and solidarity. As a social bond, it works with the value of (personal and group) attachment (translated ‘love’) and the value of (personal and group) allegiance or trust (translated ‘hope.’) p. 72 Pilch and Malina Handbook of Biblical Social Values.
I am well aware that some Christians will say they have 100 percent certainty because disciples of Jesus are blessed to receive the assurance of the truthfulness of our faith through the work of the Holy Spirit (Rom 8: 16-17; 2 Cor. 2:2). So they may appeal to what they call “supernatural certainty.” However, we cannot confuse feeling certain and being certain. There’s a difference. Being certain involves appealing to argument and evidence. When we appeal to reason and argument, while we can appeal to deductive, inductive, and abductive arguments, we won’t be able to hit 100 percent certainty. Remember, if we had a 100% doubt free belief system, there wouldn’t be any room for faith/trust in God. Any Christian that thinks they have a perfect, doubt free faith are setting themselves up for disappointment. Also, anyone who assumes apologetics is supposed to answer every single question exhaustively has misunderstood the limitations of apologetics. Thus, we if tell people faith/trust is equivalent to having absolute or almost perfect certainty, people will keep having a crisis of faith.