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By "Unbelief", we mean a state that is characterized by the absence of believe. Unbelief itself is doubt - the absence of the cardinal virtue of faith.
The devil is much more aware that he cannot oppress God's children (because it's not within his jurisdiction). God's presence is paramount here, just as in the case of Job.
For this reason, the only way which he can send forth his attack is through our thoughts. He comes through the mind, that's why this phenomenon is referred to as "The Battle Field of the Mind" - one of the books written by Joyce Meyer.
We are whom we say we are; what we believe is what works for us. Hence, the devil sows seeds of negativity in our minds.
FEAR AND UNBELIEF
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When we allow these toxic seeds to germinate in our minds through our thoughts, it threatens our faith and sends a signal of fear.
This negative signal in turn breeds Unbelief, for Unbelief and fear coexist.
With time, we begin to doubt who God is and as well as limit the majesty of God's goodness in our hearts.
2 Timothy 1:7 For God hath not given us the spirit of fear; but of power, and of love, and of a sound mind.
Fear doesn't come from God, for it's of the devil. It takes fear to respond to what the devil has in store for you.
It's in contrast to faith which comes by hearing God's Word.
UNBELIEF AND SIN
The act of unbelief is sinful, in fact, it's the master of all sins. That is, all sins are as a reaction or action of the sin of unbelief.
Let's look at how the sin of doubt which is the only unforgivable sin (sin against the Holy Spirit) is the mother of all sins.
Considering the sin of theft: you steal because you don't believe that God will supply all your needs in accordance to His riches.
With respect to avarice and greed: you ain't generous because you don't believe in what God has promised concerning those who give.
With respect to the sin of hatred, you're entangled in that bondage because you don't believe in the God whom is love Himself who has called us to love.
Therefore in general, you commit sin because you don't believe in what God's Word says about that your case.
Faith is manifested in action; in its absence, we cannot please God.
BELIEVE!
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As people of faith, we are called to walk in the path of light; to renew our minds with God's Word, that our faith might be increased and that we may grow in the knowledge of Him who has called us.
It's only in this regards that we can manifest as children.
Romans 8:19 For the earnest expectation of the creature waiteth for the manifestation of the sons of God.