The church makes a distinction between mortal sin and venial sin. His sacrifice was for the remission and forgiveness of sins, always when Christ forgives he also mentions to sin no more. 

In the ancient Church hardly any of the laity received communion because people were aware of their own sins, in fact we had a classification amongst laity. This is no medieval invention read the didascalia written as a catechism back when the Church assembled in houses. They were not allowed to partake in the Eucharist and had to stand separated from the rest of the congregation. 

Being in a state of grave sin negates the whole purpose of communion. Why do we take communion? In order to be deified(theosis) to partake in Christ's divine nature and resurrect. How can come into communion if we are attached to our worldly sins? Through confession we come into a state of grace, where we can be receptive to the gift of divine flesh and blood so we can become Christ-like.

Pick up your cross and follow.