What is a Sacrament?

 The Gospels were written by men who were among the first to have the faith[Cf. Mk 1:1 ; Jn 21:24 .] and wanted to share it with others. Having known in faith who Jesus is, they could see and make others see the traces of his mystery in all his earthly life. From the swaddling clothes of his birth to the vinegar of his Passion and the shroud of his Resurrection, everything in Jesus' life was a sign of his mystery.[Cf Lk 2:7 ; Mt 27: 48 ; Jn 20:7 .] His deeds, miracles and words all revealed that 'in him the whole fullness of deity dwells bodily.'[Col 2:9 .] His humanity appeared as 'Sacrament', that is, the sign and instrument, of his divinity and of the salvation he brings: What was visible in his earthly life leads to the invisible mystery of his divine sonship and redemptive mission"

The Church celebrates the mystery of her Lord 'until he comes,' when God will be 'everything to everyone.'[1 Cor 11:26 ; 1 Cor 15:28 .] Since the apostolic age the liturgy has been drawn toward its goal by the Spirit's groaning in the Chttp://hurch: Marana tha![1 Cor 16:22 .] The liturgy thus shares in Jesus' desire: 'I have earnestly desired to eat this Passover with you . . . until it is fulfilled in the kingdom of God.'[Lk 22:15 .] In the sacraments of Christ the Church already receives the guarantee of her inheritance and even now shares in everlasting life, while 'awaiting our blessed hope, the appearing of the glory of our great God and Savior Christ Jesus.'[Titus 2:13 .] The 'Spirit and the Bride say, 'Come . . . Come, Lord Jesus!''[Rev 22:17, 20.]

St. Thomas sums up the various aspects of sacramental signs: 'Therefore a Sacrament is a sign that commemorates what precedes it- Christ's Passion; demonstrates what is accomplished in us through Christ's Passion - grace; and prefigures what that Passion pledges to us - future glory.'[St. Thomas Aquinas, STh III, 60, 3."

  "Baptism is the sacrament of faith.[Cf. Mk 16:16 .] But faith needs the community of believers. It is only within the faith of the Church that each of the faithful can believe. The faith required for Baptism is not a perfect and mature faith, but a beginning that is called to develop. The catechumen or the godparent is asked: 'What do you ask of God's Church?' The response is: 'Faith!'"

   "St. Augustine admirably summed up this doctrine that moves us to an ever more complete participation in our Redeemer's sacrifice which we celebrate in the Eucharist:
"This wholly redeemed city, the assembly and society of the saints, is offered to God as a universal sacrifice by the high priest who in the form of a slave went so far as to offer himself for us in his Passion, to make us the Body of so great a head.... Such is the sacrifice of Christians: 'we who are many are one Body in Christ' The Church continues to reproduce this sacrifice in the Sacrament of the altar so well-known to believers wherein it is evident to them that in what she offers she herself is offered.[St. Augustine, De civ Dei, 10, 6: PL 41, 283; cf. Rom 12:5 .]"

"It is by the conversion of the bread and wine into Christ's body and blood that Christ becomes present in this sacrament. The Church Fathers strongly affirmed the faith of the Church in the efficacy of the Word of Christ and of the action of the Holy Spirit to bring about this conversion. 

Thus St. John Chrysostom declares:
It is not man that causes the things offered to become the Body and Blood of Christ, but he who was crucified for us, Christ himself. The priest, in the role of Christ, pronounces these words, but their power and grace are God's. This is my body, he says. This word transforms the things offered.[St. John Chrysostom, prod. Jud. 1:6: PG 49, 380.]

And St. Ambrose says about this conversion:
Be convinced that this is not what nature has formed, but what the blessing has consecrated. The power of the blessing prevails over that of nature, because by the blessing nature itself is changed.... Could not Christ's word, which can make from nothing what did not exist, change existing things into what they were not before? It is no less a feat to give things their original nature than to change their nature.[St. Ambrose, De myst. 9, 50; 52: PL 16, 405-407.] "

Catechism of the Catholic Church # 1368-1374