We all get sad sometimes, and that’s a normal human feeling. However, we need to remember that Shaytān can sometimes use sadness to defeat the believer, turn him/her against Allāh, lead them astray and/or turn them to disbelief entirely.
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The reality is, we all go through certain situations that make us sad. But Shaytān, if he fails to make people commit sins, he would work on making them ungrateful to Allāh and hopeless of any change in their life. Sadness is a great opportunity for him to do that. It is ok to be sad, but if you remain in sadness...