'And if anyone,' our Lord says, 'shall compel you to go a mile, go with him two' (Mt. 5:41); so that you may not follow him as a slave, but may as a free man go before him, showing yourself in all things kindly disposed and useful to your neighbor, not regarding their evil intentions, but performing your kind services, likening your self to the Father.
— From St. Irenaeus (The Ante-Nicene Fathers: Vol. I; Eerdmans pg. 477)
As two pieces of wax fused together make one, so he who receives Holy Communion is so united with Christ that Christ is in him and he is in Christ.