Today's section of St John's Gospel is the start of the 'bread of life' discourse.
The Greek, Latin and Knox translations of John 6 can be found over at New Advent. And you can listen to it in Latin here (from 3.18) and the Greek from here.
28 Dixerunt ergo ad eum: Quid faciemus ut operemur opera Dei? 29 Respondit Jesus, et dixit eis: Hoc est opus Dei, ut credatis in eum quem misit ille.30 Dixerunt ergo ei: Quod ergo tu facis signum ut videamus et credamus tibi? quid operaris? 31 Patres nostri manducaverunt manna in deserto, sicut scriptum est: Panem de cælo dedit eis manducare. 32 Dixit ergo eis Jesus: Amen, amen dico vobis: non Moyses dedit vobis panem de cælo, sed Pater meus dat vobis panem de cælo verum. 33 Panis enim Dei est, qui de cælo descendit, et dat vitam mundo. 34 Dixerunt ergo ad eum: Domine, semper da nobis panem hunc. 35 Dixit autem eis Jesus: Ego sum panis vitæ: qui venit ad me, non esuriet, et qui credit in me, non sitiet umquam. 36 Sed dixi vobis quia et vidistis me, et non creditis. 37 Omne quod dat mihi Pater, ad me veniet: et eum qui venit ad me, non ejiciam foras: 38 quia descendi de cælo, non ut faciam voluntatem meam, sed voluntatem ejus qui misit me. 39 Hæc est autem voluntas ejus qui misit me, Patris: ut omne quod dedit mihi, non perdam ex eo, sed resuscitem illud in novissimo die. 40 Hæc est autem voluntas Patris mei, qui misit me: ut omnis qui videt Filium et credit in eum, habeat vitam æternam, et ego resuscitabo eum in novissimo die.
 They said therefore unto him: What shall we do, that we may work the works of God?  Jesus answered, and said to them: This is the work of God, that you believe in him whom he hath sent.  They said therefore to him: What sign therefore dost thou shew, that we may see, and may believe thee? What dost thou work?  Our fathers did eat manna in the desert, as it is written: He gave them bread from heaven to eat.  Then Jesus said to them: Amen, amen I say to you; Moses gave you not bread from heaven, but my Father giveth you the true bread from heaven.  For the bread of God is that which cometh down from heaven, and giveth life to the world.  They said therefore unto him: Lord, give us always this bread.  And Jesus said to them: I am the bread of life: he that cometh to me shall not hunger: and he that believeth in me shall never thirst.  But I said unto you, that you also have seen me, and you believe not.  All that the Father giveth to me shall come to me; and him that cometh to me, I will not cast out.  Because I came down from heaven, not to do my own will, but the will of him that sent me.  Now this is the will of the Father who sent me: that of all that he hath given me, I should lose nothing; but should raise it up again in the last day.  And this is the will of my Father that sent me: that every one who seeth the Son, and believeth in him, may have life everlasting, and I will raise him up in the last day.
The crowd, predictably, demand yet another miracle. Instead, as the Catena Aurea extracts from the Fathers point out, Jesus confronts them with his own divinity, in which we must believe:
CHRYS. Our Lord now proceeds to set forth mysteries; and first speaks of His Divinity: And Jesus said to them, I am the bread of life. He does not say this of His body, for He speaks of that at the end; The bread that I will give you is My flesh. Here He is speaking of His Divinity. The flesh is bread, by virtue of the Word; this bread is heavenly bread, on account of the Spirit which dwells in it.
THEOPHYL. He does not say, I am the bread of nourishment, but of life, for, whereas all things brought death, Christ has quickened us by Himself. But the life here, is not our common life, but that which is not cut short by death: He that comes to Me shall never hunger; and, He that believes in Me shall never thirst.
AUG. He that comes to Me, i.e. that believes in Me, shall never hunger, has the same meaning as shall never thirst; both signifying that eternal society, where there is no want.
Belief though, is not a work of the human intellect alone, but rather requires grace:
CHRYS. The expression, that the Father gives Me, shows that it is no accident whether a man believes or not, and that belief is not the work of human cogitation, but requires a revelation from on high, and a mind devout enough to receive the revelation. Not that they are free from blame, whom the Father does not give, for they are deficient even in that which lies in their own power, the will to believe. This is a virtual rebuke to their unbelief, as it shows that whoever does not believe in Him, transgresses the Father's will. Paul, however, says, that He gives them up to the Father: When He shall have given up the kingdom to God, even the Father. But as the Father, in giving, does not take from Himself, so neither does the Son when He gives up. The Son is said to give up to the Father, because we are brought to the Father by Him. And of the Father at the same time we read, By Whom you were called to the fellowship of His Son. Whoever then, our Lord says, comes to Me, shall be saved, for to save such I took up flesh: For I came down from heaven not to do Mine own will, but the will of Him that sent Me. But what? Have you one will, He another? No, certainly. Mark what He says afterwards; And this is the will of Him that sent Me, that every one which sees the Son, and believes in Him, should have everlasting life. And this is the Son's will too; For the Son quickens whom He will. He says then, I came to do nothing but what the Father wills, for I have no will distinct from My Father's: all things that the Father has are Mine. But this not now: He reserves these higher truths for the end of His ministry.
AUG. They therefore who by God s unerring providence are foreknown, and predestined, called, justified, glorified, even before their new birth, or before they are born at all, are already the sons of God, and cannot possibly perish; these are they who truly come to Christ. By Him there is given also perseverance in good to the end; which is given only to those who will not perish. Those who do not persevere will perish."
The next set of lectio notes can be found here.