In classical electromagnetism, Gauss’s flux theorem in integral form states that the electric flux through a closed surface is proportional to the net electric charge enclosed by that surface. Gauss’s flux theorem in integral form is originally derived from Coulomb’s law of electrostatics. However, electrodynamics wrongly extends Gauss’s flux theorem to time-varying electric fields without additional theoretical and experimental justification, while confining Coulomb’s law to the scope of electrostatics. Through concise and rigorous mathematical derivations, this paper reveals that the integral form of Gauss’s flux theorem in classical electromagnetism does not hold in time-varying electric fields. Furthermore, it is demonstrated that Gauss’s flux theorem in differential form, which is expressed in terms of the flux-density divergence or the nabla-operator divergence, also does not hold in time-varying electric fields. Gauss’s flux theorem does not hold in time-varying electric fields, which poses a serious challenge to Maxwell’s equations and theoretical physics. The inapplicability of divergence to time-varying vectors also presents a significant challenge to fundamental mathematics.