Faith is one of those words that is difficult to tie down to one simple definition.
According to a modern dictionary, faith is "unquestioning belief that does not require proof or evidence." As it regards the faith found in the Bible, this is simply not true. A. A. Hodge truly said, "Faith must have adequate evidence, else it is mere superstition." Faith is not the opposite of fact or of scientific knowledge. The evidences of faith may operate differently than those of science, but they are there.
Someone else has come up with an acronym for FAITH:
This is an good illustration to use when teaching about faith, but as with most definitions of this kind, it still comes far short.
Many Bible students will give Hebrews 11:1 as the definition for faith and leave it at that. Hebrews 11:1 states, "Now faith is the substance of things hoped for, the evidence of things not seen." However, even though this is the form defintions are given in, this verse is more of an expression of the power and importance of faith than it is a definition of faith. In fact, one already needs to know what faith is before this verse will make much sense to them. This makes this verse a frustrating answer to someone who is trying to get a handle on the foundational meaning of faith.
If you want a short definition of faith, it could be this: taking God at His word. It is true that our faith is in God. But we do not properly know the God we should believe in or know how to believe in Him unless He tells us in His word. This is why Paul says in Romans 10:17, "So then faith cometh by hearing, and hearing by the word of God." Biblical faith is not an "unquestioning belief that does not require proof or evidence" (Webster's New World Dictionary). It is full confidence in God's word. FAITH accepts God's word (His promises and His warnings) as FACT and acts accordingly. Since there are many evidences that the word of God is true, this is not a blind leap of faith. It is rather an intelligent, holy reaction to the wondrous words of God.
Now it is true that faith is much more than a mental acceptance of something as true; it also involves a trust in or reliance on that something. The Reformers of the sixteenth and seventeenth centuries had a good approach to faith. They taught what they called the three aspects of faith. Here are the three aspects of faith:
Knowledge. Faith begins with a knowledge of what it is that should be believed. For instance, if someone knows that the gospel of Christ refers to the death, burial, and resurrection of Christ according to the scriptures (see 1 Corinthians 15:1-4), they have knowledge. However, it is possible for someone to know what the gospel is without believing it to be true. Therefore:
Assent. Knowledge is not enough. The person must also believe that the object of faith is true. To reach this level of faith, the person must know what the gospel is (knowledge) and believe it to be true (assent). But this is still not enough for salvation. That takes us to the third aspect of faith.
Trust. Trust refers to a personal commitment to and reliance upon an object of faith. In salvation, the sinner must know that Jesus died for him and rose again from the dead (knowledge) and he must accept that these facts are true (assent). However, he is still not saved until he relies on these facts as the basis for his personal salvation. Let me give you a couple of illustrations to help you understand these three aspects.
Let us say that you are visiting someone's home and they ask you to sit down. First, you look over and acknowledge that there is indeed a chair. This is knowledge. Second, you accept the fact that you could sit in this chair and it would hold you up. That is assent. Finally, you walk over to the chair and sit down in it. That is trust. It is in this third aspect of faith that you exercise and complete your faith.
Another example applies the three aspects to one of the promises given to believers. The Bible tells us that all things work together for good (Romans 8:28). Knowing that this teaching is in the Bible is knowledge. Accepting that the fact is true is assent. However, many people take these first two steps and choke at the third one, for it is in trust that we commit our own circumstances to this truth and believe that it refers to us and our own life. We are to "walk by faith" (2 Corinthians 5:7). That means that we are to live our life in such a way that the promises and warnings of God are fully accepted and applied to our own life and our particular circumstances.
LARGER RELATIONSHIPS OF FAITH:
On a grander scale, faith is a link in a chain that has a link before it and a link after it. Allow me to briefly introduce these connections:
Founded on the Word. Biblical faith is not a mystical belief in God and in other principles based on inner knowledge, personal revelations, or human reasoning. It is not a leap in the dark. Biblical faith firmly stands on the words of Almighty God. When Romans 10:17 is distilled, it teaches us that faith comes by the word of God. Any faith not founded on the word of God is not Biblical faith.
Faith. This includes all the aspects of faith we have already spoken of.
Functions through Works. Do not misunderstand. Faith is not works and works are not faith. Salvation is by faith alone without works (Ephesians 2:8-9). However, the natural result of genuine faith is works. Jesus said, "He that believeth on me, the works that I do shall he do also; and greater works than these shall he do" (John 14:12). Paul referred to the "faith which worketh by love" (Galatians 5:6) and spoke to the Thessalonians of the "work of faith" (1 Thessalonians 1:3; 2 Thessalonians 1:11). He told Titus: "these things I will that thou affirm constantly, that they which have believed in God might be careful to maintain good works" (Titus 3:8). These and other places show us that works are the proper result of true faith. There are indeed many complexities concerning this connection that cannot be gone into here. It is enough to know that there is a connection. Let me give you one illustration of how this works in life.
Let us say that you are going to find and purchase a pair of shoes and that a friend has told you that the best shoes on the market are the Boing! brand. You go to the shoe store and find two pairs of shoes that look good to you. One is the Boing! brand and one is not. However, because of your friend's recommendation, you purchase the Boing! brand. Now, where are the three connections of faith mentioned above? First, you have the recommendation of your friend. Your faith is founded on his word. Second, you believe your friend. That is faith. Finally, you bought the brand of shoes recommended by your friend. That is, your faith functioned through works (you bought the shoes). This same pattern can be found in every act of faith in our Christian life. We are saved by faith; but we must also live the Christian life by faith.