Question: I have a question that I hope you can answer for me, please: Is lack of confidence or social anxiety a sin?
Response: Lack of confidence may simply be called “fear.” Social anxiety can be simplified to “anxiety,” regardless of what causes it. In the trials and circumstances that come before us, we may be tempted to yield to fear and anxiety in a sinful manner. On some occasions, the Lord may want us to stand and see His salvation. At other times, we clearly know to remove ourselves from the situation. We all face fear and anxiety to one degree or another, but do we allow them to cause us to flee from circumstances under which we know the Lord would want us to remain for His purposes?
In the book of Esther, a royal decree has been issued that will bring a massacre of all the Jews in Persia. Queen Esther is anxious and fearful because she was asked by her uncle Mordecai to risk her life on behalf of her people. The possibility of death and the unknown is certainly cause for fear and anxiety for anyone! Mordecai specifically tells Esther that “[I]f thou altogether holdest thy peace at this time, then shall...deliverance arise to the Jews from another place; but thou and thy father’s house shall be destroyed: and who knoweth whether thou art come to the kingdom for such a time as this?” (4:14).
Had Esther yielded to her fear and anxiety and fled or refused to follow through, her action, it is directly implied, would have been sinful and would have resulted in tragedy for her. Although fear and anxiety are part of the human condition, it only becomes sin when we yield to the temptation to flee from challenges that we know God is allowing.
For those who are the children of God, our very necessities such as food and clothing are not to be the cause of worry. Jesus points to His creation, showing that our Heavenly Father knows our needs, is concerned with them and for us (Mt 6:25-31). If God cares for grass, flowers, and birds, why shouldn’t He also care for His children who are created in His image? Chapter 6 ends with the admonition to “seek ye first the kingdom of God and his righteousness, and all these things shall be added unto you” (v. 33). Rather than yielding to our fears and anxieties, we must turn to God and His Word. The promise of our Lord is that “all these things shall be added unto you.” What things? The necessary things of life.
To yield to our fears and anxieties is to doubt the promise of God. That common scenario of mankind showed up in the Garden when Satan asked, “Yea, hath God said...?” (Gn 3:1). Yes, it was doubt that led to the sin that brought in the curse.
You speak of social anxiety. Interaction with other people can certainly bring anxiety along with the temptation to avoid or flee. But we are called to be ambassadors for Christ (2 Cor:5:20) and that involves social interaction with others.
Finally, James:1:2-4 exhorts, “My brethren, count it all joy when ye fall into divers temptations; knowing this, that the trying of your faith worketh patience. But let patience have her perfect work, that ye may be perfect and entire, wanting nothing.”