Doubt is a lack in the ability to discern what is proper and what is wrong in our actions. As Christians, we should have our senses trained to discern the difference between actions that are good and actions that are wrong.
But the solid food is of [the] mature, the ones through the exercise train the senses, having discernment toward both good and wrong – Hebrews 5:14
Because of this inability to discern between right and wrong the believer hesitates and will not act out from faith.
but let him ask in faith, not while doubting. For the one doubting is like a wave of the sea driven by the wind and tossed – James 1:6
Anything done by a Christian that is not out from faith is sin, which means that an action that normally would not be sin, such as eating and drinking, can become sin to the one who is not discerning what is proper in contrast to what is wrong. How can eating be wrong? Questioning whether God allows you to eat it without violating His standard and therefore causing a conscience issue because our action is not lining up with what we think is God’s expectation of us.
But the one doubting, if perhaps, he should eat is condemned because it is not out from faith. And all which is not out from faith is sin. - Romans 4:23
The righteous live out from faith. Satan does not want us to live by faith because it means we will be applying the promises God has given to the Church, training our senses to discern what is proper and what is wrong, and living a life of righteousness out from faith, which makes it very difficult for Satan to manipulate us. Satan wants Christians to be ignorant and therefore doubting so that we sin. He wants us to mix up Old and New Testament information and not make proper distinctions, which will always lead to sin. One who is sinning is not fellowshipping with God and therefore is not able to use the benefits of salvation, which includes having victory over Satan.
Remember, as part of our salvation we are sons; therefore, we are expected to act as mature ones, not children. Children are inarticulate babblers - ones who cannot properly explain what they know or how to correctly use knowledge.
When I was an inarticulate babbler, I spoke as an inarticulate babbler, I framed the mind as an inarticulate babbler, I reasoned as an inarticulate babbler. But when I became a man I rendered ineffective the things of the inarticulate babbler - 1 Corinthians 13:11
Study God's Word, be around other saints, grow together while helping each other, use the knowledge you have while taking the time to discern what is proper and what is wrong. And as your knowledge grows don't be so stubborn that you do not grow with it. In each situation we face we can discern what is proper for us as a child of God and what would be wrong or contrary to what God expects of us as His children. How am I showing a proper opinion of God? How am I showing a proper opinion of who I am in Christ? Yes, this takes time and effort, but both are well spent.
A good example of this today is whether or not to wear a mask. In some situations it is proper, in others it is not. We know masks do nothing for us and do not protect anyone. Yet, by being willing to submit to wearing a mask for the sake of others we can show a proper opinion of who we are in Christ? And of course, in other situations we should refuse to wear a mask because it would not show a proper opinion of God. We need to have our senses trained to know when we are not scaring the sheep and when we are standing against a wolf.