If the resultant of all external forces acting on a system of particles is zero, then from an inertial frame one can surely say that

The net external force acting on a system of particles is given by the rate of change of the total linear momentum of the system, expressed as $\vec{F}_{ext} = \dfrac{d\vec{P}}{dt}$.

Given that the resultant of all external forces is zero, $\vec{F}_{ext} = 0$.

Substituting this into the equation, we get $\dfrac{d\vec{P}}{dt} = 0$.

Integrating with respect to time, we find that the total linear momentum $\vec{P}$ of the system remains constant over time. This is the principle of conservation of linear momentum.

Answer: Linear momentum of the system does not change in time