  Non-conserved composite operators like the quark axial current have divergent
matrix elements therefore must be renormalized. We explore how this can be done
in quark model calculations where the systematic procedure of dimensional
regularization and minimal subtraction is not applicable. We propose a most
natural and convenient regularization scheme of cutting the intermediate quark
states over which we sum in loop diagram calculations at a certain energy. We
show that this scheme works perfectly for the quark axial current and we obtain
the quark spin contribution to the proton spin: $\Delta_u=0.82$,
$\Delta_d=-0.43$, $\Delta_s=-0.10$, which is in excellent agreement with
experiments.
