The Eucharist is the source and summit of the Christian life, because it is Jesus wholly present: body, blood, soul and divinity. 

The Eucharist is also a sacrifice because it makes Jesus’ unique sacrifice on the cross present to us.  At every Mass Jesus unites our sufferings, prayer, and work with the total gift of himself to the Father. 

In Holy Communion we take Jesus into our bodies under the appearance of bread and wine, but he transforms us into himself. The whole Christ is really, truly, and substantially present in the Holy Eucharist. We use the words "really, truly, and substantially" to describe Christ's presence in the Holy Eucharist in order to distinguish Our Lord's teaching from that of mere men who falsely teach that the Holy Eucharist is only a sign or figure of Christ, or that He is present only by His power.