At first glance, it seems apparent that the opposite of “faith” is “doubt”. This is true but it does seem to require some further explanation. In the new testament, there are two different Greek words translated doubt and they are different enough to need further explanation. One is in fact the opposite of faith while the other is indicative of a faith that is still in need of strengthening.
The first of these two words can be illustrated by the story of Peter walking on the water and the second in Jesus’ teaching on faith that moves mountains. In order to get to the distinction between the two types of doubt, let’s start in Hebrews 11:1
Now faith is the certainty of things hoped for, a proof of things not seen. (NASB)
In several places the idea of faith is related to standing or taking a stand in the new testament. When we read “certainty” (sometimes translated “substance”) we are seeing a Greek word that has the connotation of an underlying thing upon which one stands. You could say that faith gives you a solid platform or substance on which to stand with certainty.
In the story of Peter walking on the water found in Matthew 14, we know that the disciples left to cross the sea on their boat without Jesus. A storm came up battering the boat and Jesus approached them walking on the water. Peter cried out to Jesus and ultimately left the boat walking to Jesus on the water. The next two verses show us what happened when Peter was distracted from his focus on Jesus by the wind and waves, Matthew 14:30-31
But seeing the wind, he became frightened, and when he began to sink, he cried out, saying, “Lord, save me!” Immediately Jesus reached out with His hand and took hold of him, and *said to him, “You of little faith, why did you doubt?” (NASB)
The word recorded here for doubt is related to the word “certainty” in Hebrews 11:1. Where certainty describes a solid foundation upon which to stand (or a single place to stand), doubt literally means “two-stand” or “double-stand”. In other words Jesus commented on Peter’s “little faith” by asking why he allowed himself to be distracted by an alternative truth or something else upon which to stand. Jesus did not chide Peter for a complete lack of faith, He simply pointed out that Peter’s small, or not fully developed, faith allowed him to become distracted and contemplate the impossibility of what he was currently doing. In other words Peter’s doubt can be defined as a momentary distraction away from Jesus, a temporary loss of focus on what Jesus had plainly instructed him that he could and should do. Fortunately Jesus was quick to grab Peter and rescue him from the distraction of wind and wave.
In contrast to this type of doubt is complete different Greek word found in Mark 11:23.
Truly I say to you, whoever says to this mountain, ‘Be taken up and thrown into the sea,’ and does not doubt in his heart, but believes that what he says is going to happen, it will be granted to him. (NASB)
Here, the doubt that can be found in the the heard is a compound word that describes the means or process by which a judgement or decision is made. This word for doubt is not a temporary lack of focus or distraction. The doubt that causes a mountain to remain unmoved is a conscious decision, a weighing of evidence and rendering a judgement. This type of doubt is a decision to give up or reject what God has said. It is this decision that disqualifies someone from seeing the proverbial mountain thrown into the sea.
In light of these two definitions of doubt, it might be more accurate to say that the opposite of faith is quit.