Is one who takes a paper from his non-Muslim teacher on which a king and a devil are drawn in a cartoonish, mocking manner, and does not denounce it, sinful?

The questioner is advised to stay away from obsessive whispering (waswasah) and from being led by Satan in matters of mockery. As for denouncing such acts, it is a religious requirement, and a Muslim sins if they do not do so when they are able. However, if one perceives that the benefit of winning over the disbeliever's heart necessitates not denouncing the act or delaying it, then there is no harm in that, in order to achieve a greater benefit, which is the disbeliever's embracing of Islam.