Is it shirk or an act of kufr to say to someone "No-one can touch you"?
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
This statement does not involve Kufr (disbelief) or Shirk (polytheism) at all as long as the sayer means that no created being is able to harm this person except by the will of Allaah, The Exalted. On the contrary, this would be an affirmation of Tawheed (faith in the Oneness of Allaah). Allaah, The Exalted, says (what means): {…But they do not harm anyone through it except by permission of Allaah…}[Quran 2:102]
Moreover, Ibn ‘Abbaas reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: “…Know that if all the people were to gather together to do something to benefit you, you would never get any benefit unless Allaah has written for you something. And if they were to gather to do something to harm you, nothing would harm you except that which Allaah has written for you. The pens are lifted and the scrolls have dried.” [Ahmad and At-Tirmithi]
Also, if he means that he will strive in defending him (that person) in such a way that none can harm him, there is nothing wrong with that either, and it is not considered an act of disbelief or Shirk.
Allaah Knows best.
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