Peace be with you; as the Father has sent Me, I also send you.” And when He had said this, He breathed on them and said to them, “Receive the Holy Spirit. If you forgive the sins of any, their sins have been forgiven them; if you retain the sins of any, they have been retained. (John 20:21-23)
Why do I have to receive the sacrament of Confession after committing a mortal sin before I can receive the Eucharist?
When in a state of mortal sin, the sin corrupts the purity of our soul and prevents us from receiving the grace that the Eucharist brings to us by consuming the Body and Blood of Christ. When we receive Jesus we have to have our whole hearts, whole minds, and whole souls available to him; but grave sin on our conscience and within our hearts acts as a barrier that does not allow the light of Christ to penetrate through, and then we cannot share that light with others. Only the sacrament of Confession can tear down that barrier and enable us to receive the grace promised to us by the Eucharist.