"Proclaim the death of the Lord until He comes"

He had one other to send, a beloved son.
He sent him to them last of all, thinking, 'They will respect my son.'
But those tenants said to one another, 'This is the heir.
Come, let us kill him, and the inheritance will be ours.'

-- Mk 12:1-12

Holy Communion separates us from sin. The body of Christ we receive in Holy Communion is "given up for us," and the blood we drink "shed for the many for the forgiveness of sins." For this reason the Eucharist cannot unite us to Christ without at the same time cleansing us from past sins and preserving us from future sins:

For as often as we eat this bread and drink the cup, we proclaim the death of the Lord. If we proclaim the Lord's death, we proclaim the forgiveness of sins. If, as often as his blood is poured out, it is poured for the forgiveness of sins, I should always receive it, so that it may always forgive my sins. Because I always sin, I should always have a remedy.

-- CCC 1393

As early as the second century we have the witness of St. Justin Martyr for the basic lines of the order of the Eucharistic celebration. They have stayed the same until our own day for all the great liturgical families. St. Justin wrote to the pagan emperor Antoninus Pius (138-161) around the year 155, explaining what Christians did:

On the day we call the day of the sun, all who dwell in the city or country gather in the same place.

The memoirs of the apostles and the writings of the prophets are read, as much as time permits.

When the reader has finished, he who presides over those gathered admonishes and challenges them to imitate these beautiful things.

Then we all rise together and offer prayers* for ourselves . . .and for all others, wherever they may be, so that we may be found righteous by our life and actions, and faithful to the commandments, so as to obtain eternal salvation.

When the prayers are concluded we exchange the kiss.

Then someone brings bread and a cup of water and wine mixed together to him who presides over the brethren.

He takes them and offers praise and glory to the Father of the universe, through the name of the Son and of the Holy Spirit and for a considerable time he gives thanks (in Greek: eucharistian) that we have been judged worthy of these gifts.

When he has concluded the prayers and thanksgivings, all present give voice to an acclamation by saying: 'Amen.'

When he who presides has given thanks and the people have responded, those whom we call deacons give to those present the "eucharisted" bread, wine and water and take them to those who are absent.

-- CCC 1345