Mark 3:21 mentions some persons that said Jesus had lost his senses. Does the text say Jesus' kinsmen said he was mentally ill or was it others outside the family? Let's find out.
The author of Mark divides his narrative into two scenes in 3:20-35. One scene takes place where Jesus resides (verses 20, 22-35) the other is where his "kinsmen" are to be found (verse 21). It was these kinsmen, his mother and brothers, who "went out to take custody of him [Jesus]." One does not take custody of someone who is well. They felt Jesus had lost his mind. In order to take custody of him they had to go where he was residing. As a result, the text says, ". . . his own kinsmen . . . went out to take custody of him . . . ." In verse 31 the narrative describes their arrival at the place where Jesus was staying.
Jesus is told that his mother and brothers are outside looking for him. His Answer, "Who are my mother and my brothers? . . . For whoever does the will of God, he is my brother and sister and mother," (verses 33-35) shows that he was hostile to their attempt to see him. There could be but one reason. His family, mother, brothers, and sisters, believed him to be mentally ill and had not come simply to see him, but had come to seize him.