What causes the sin of fear? The second item listed in Revelation 21:8 points to the answer—unbelief. Psalm 78:13-16 gives an example of how this works:
For he divided the sea before them and led them through! The water stood up like walls beside them! In the daytime he led them by a cloud, and at night by a pillar of fire. He split open the rocks in the wilderness to give them plenty of water, as from a gushing spring. He made streams pour from the rock, making the waters flow down like a river! (New Living Translation [NLT])
Listed are miracles God performed to provide for the needs of His people. Continuing in Psalm 78:17-21, we see Israel's response to God's clear display of His love and care for them:
Yet they kept on with their sin, rebelling against the Most High in the desert. They willfully tested God in their hearts, demanding the foods they craved. They even spoke against God himself, saying, "God can't give us food in the desert. Yes, he can strike a rock so water gushes out, but he can't give his people bread and meat." When the Lord heard them, he was angry. The fire of his wrath burned against Jacob. Yes, his anger rose against Israel. . . . (NLT)
Even though God had provided water in abundance, they were afraid that He would not provide bread and meat. What was the basis, the cause, for this fear? The answer follows in Psalm 78:22 (NLT): ". . . for they did not believe God or trust him to care for them." Amazing! After all God did for them, they still could not muster the necessary faith and trust in God and His love for them (Hebrews 4:2). When push comes to shove, do we believe God's promises? Do we trust in the extent of His care and love for us (John 17:23) and that He will come to our aid (Psalm 34:19)?
What was God's response to the Israelites' display of fear? "The fire of his wrath burned against Jacob" (Psalm 78:21, NLT), foreshadowing Revelation 21:8. Fire is the response to fear from the God who does not change (Malachi 3:6).
Fear, then, is the result of a lack of faith, not believing in God's power and especially His love and willingness to act on our behalf. The fearful are that way because they lack faith. Christ reveals this connection in Mark 4:40: "But He said to them, 'Why are you so fearful? How is it that you have no faith?'"
When Christ saw fear, His immediate response was to question that person's faith. Fear is a very human reaction. As Christ indicates, how we respond depends on our degree of faith. When faith is weak or non-existent, fear becomes the controlling factor rather than faith. We begin to live by sight and not by faith (II Corinthians 5:7), and without faith, we cannot be saved: "And to whom did He swear that they would not enter His rest, but to those who did not obey? So we see that they could not enter in because of unbelief" (Hebrews 3:18-19).
Our rest is God's Kingdom. Just as a lack of faith—unbelief—barred ancient Israel from entering their rest, a lack of faith can bar us from entering ours, keeping us out of the Family of God. Without faith, it is impossible to please God (Hebrews 11:6). We are seeing that sin is not the result of weakness as we normally think of it, as in we are "weak in the flesh." Yes, there is weakness, but what is the root of sin—its cause? When looked at closely, the root of sin is "unbelief," as Israel's example illustrates. Unbelief—a lack of faith—causes and leads to producing sin in our lives, pointing to our real foundational weakness: a lack of faith. This is Paul's message in Romans 14:23:" . . . for whatever is not from faith is sin."