  We study the small perturbations of the $1+3$-dimensional Milne model for the
Einstein-Klein-Gordon (EKG) system. We prove the nonlinear future stability,
and show that the perturbed spacetimes are future causally geodesically
complete. For the proof, we work within the constant mean curvature (CMC) gauge
and focus on the $1+3$ splitting of the Bianchi-Klein-Gordon equations.
Moreover, we treat the Bianchi-Klein-Gordon equations as evolution equations
and establish the energy scheme in the sense that we only commute the
Bianchi-Klein-Gordon equations with the spatially covariant derivatives while
the normal derivative is not allowed.
