Fear, worry, and doubt are un-Christian practices.
How can I say this? Because Jesus says this.
Jesus tells us not to worry. If we ignore him and practice worrying, then we are disobedient to Jesus.
Or, to put it another way, worry is a sin.
When we worry we are saying that we doubt that God is in control. We give power to our fear and we act as if God isn't big enough to handle our life.
Jesus also tells us not to be afraid. He tells us not to fear men, who can only destroy our body but not our soul. He even tells us not to fear Him. His Apostles remind us in the New Testament that perfect love casts out fear, and that God has not given us a spirit of fear, but of power and of love and of a sound mind.
So, if we are ruled by our fears, we are not behaving like a Christian.
Doubt is a natural human emotion, just like fear and worry. But like those other emotions, doubt isn't something that a follower of Jesus can practice.
If we are abiding in Christ, then we cannot doubt Him. If we go all in with Jesus, then that means we place all of our hope in Him, and if we do that we are not doubting, but trusting.
Of course, none of us ever perfectly avoids these three things - fear, worry and doubt - but the point is that we do not practice these things. We don't allow fear, worry and doubt to remain. We cast those things out quickly and we refuse to be ruled by those because Jesus is our King and we are submitted to Him.
If we are abiding in Jesus, then we are practicing faith, hope and love, not fear worry and doubt.
Let's continue to walk in our new nature, which is Christ in us.