I think it's a useful exercise to define faith for ourselves. What exactly should we mean when we use the term faith. What role does faith play? Is it meant to be an excuse for believing anything we want? Is it a get out of jail free card for anything we choose to promote as truth? Does faith trump truth? Some outside of the Church might think we believe all of these things about faith. Maybe you believe all of them. A definition of faith like that described is dangerous though. It promotes division instead of unity, foolishness instead of wisdom, and personal desires instead of real truth.
We can start by outlining what faith isn’t. Faith doesn’t give you any supernatural powers yourself. You can’t just turn on the faith spigot and get rich, get well, or, dare I say it, move mountains1. Faith in scripture is about a specific type of belief. It's a belief in God and God’s power. It’s purpose is to move you into alignment with God not to serve your own purposes.2 A foundational passage on faith in Scripture is Hebrews 11. In it a list of heroes of faith are listed. The one common theme in all of them is that their faith moved them to obedience.
Matt 17:14-20 is an oft quoted scripture to support the power of faith yet a closer examination of the account in Mark 9 indicates the the problem was less the quantity of faith than the fact that the faith was completely missing, as exhibited by the fact that they had not succeeded. Further Jesus comments regarding the need for prayer reveals that they had attempted the task under their own power rather than seeking to drive the demon out through an appeal to God. This would be yet another example of a lack of obedience caused by a lack of faith.
You can have “faith” in a lot of things and be completely and provably wrong. Faith in something untrue isn’t useful, it’s harmful. As a specific example, let’s say you have faith that homeopathic medicine is just as powerful as the original undiluted medicine for treating illness. You would be wrong.3 You would continue to be wrong no matter how much faith you had. In some cases you would be wrong and dead no matter how much you believe in the usefulness of homeopathic remedies.
Faith isn’t meant to deter you from study or excuse a lack of study. The statement, “Well that’s just what I believe.” isn’t, contrary to popular opinion, a valid excuse for a Christian's ignorance. Remember, Peter said we should be prepared to give a reason for the hope that is in us.4 Our faith is meant to be based on Scripture not on our own desires. If two Christians have faith in two contradictory things, then one of them is wrong. It is in their best interest to figure out which one it is. God doesn’t adjust Himself to match your faith beause He is unchanging. Our faith should instead lead us to adjust to Him.
Christian faith should be limited in the category of things we apply it to. What do I mean by that? I mean that faith doesn’t apply to provable facts. Faith isn’t necessary when the answer is demonstratable. Gravity doesn’t require faith because we can demonstrate its existence. Whether the earth revolves around the sun? No faith required. Is the earth round? Provably true.
The definition of faith is simple. For the Christian, faith should apply to the things that cannot be proven.5 Faith enables a yes or no answer for questions that are fundamentally unprovable. Does God exist? The only possible way to answer with a yes or no is through faith. Is there such a thing as sin? Only answerable through faith. Does God intervene in human affairs? You get the idea.
Faith misapplied leads you astray into a world where you are the arbiter of truth and not God. You decide what you believe instead of basing it on God’s Creation, His Scripture and the Holy Spirit. Taken too far, faith may lead you to bend and twist or even ignore Scripture to further your own belief. For the Christian, that is highly dangerous territory.6
Make sure that you are leaning on your faith in God and not your ability to imagine a god. Use faith to support your belief in things that can not be seen or proven. Above all, don't let your desire to believe in something hide the truth. Faith is a tool to motivate obedience not an excuse to be ignorant.
1: Matt 17:14-20 - https://www.bible.com/bible/59/matt.17.14-20.esv
He said to them, “Because of your little faith. For truly, I say to you, if you have faith like a grain of mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move, and nothing will be impossible for you.”
2: Heb 11 - https://www.bible.com/bible/59/heb.11.esv A passage listing heroes of the faith from the Old Testament.
3: Sutdies on Homeopathy - http://rationalwiki.org/wiki/Evidence_for_the_effectiveness_of_homeopathy A Collection of studies on the effectiveness of homeopathic remedies.
4: 1 Peter 3:15 - https://www.bible.com/bible/59/1Pe.3.15.esv but in your hearts honor Christ the Lord as holy, always being prepared to make a defense to anyone who asks you for a reason for the hope that is in you; yet do it with gentleness and respect,
5: Heb 11:1 - https://www.bible.com/bible/59/heb.11.1.esv Now faith is the assurance of things hoped for, the conviction of things not seen.
6: 2 Peter 3:16 - https://www.bible.com/bible/59/2pe.3.16.esv as he does in all his letters when he speaks in them of these matters. There are some things in them that are hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other Scriptures.