How does it change into Jesus

Cyril of Jerusalem
"The bread and the wine of the Eucharist before the holy invocation of the adorable Trinity were simple bread and wine, but the invocation having been made, the bread becomes the body of Christ and the wine the blood of Christ" (Catechetical Lectures 19:7 [A.D. 350]).

Augustine
"I promised you [new Christians], who have now been baptized, a sermon in which I would explain the sacrament of the Lord’s Table. . . . That bread which you see on the altar, having been sanctified by the word of God, is the body of Christ. That chalice, or rather, what is in that chalice, having been sanctified by the word of God, is the blood of Christ" (Sermons 227 [A.D. 411]).

Theodore of Mopsuestia
"When [Christ] gave the bread he did not say, ‘This is the symbol of my body,’ but, ‘This is my body.’ In the same way, when he gave the cup of his blood he did not say, ‘This is the symbol of my blood,’ but, ‘This is my blood’; for he wanted us to look upon the [Eucharistic elements] after their reception of grace and the coming of the Holy Spirit not according to their nature, but receive them as they are, the body and blood of our Lord. We ought . . . not regard [the elements] merely as bread and cup, but as the body and blood of the Lord, into which they were transformed by the descent of the Holy Spirit" (Catechetical Homilies 5:1 [A.D. 405]).
 

Only through faith can it be understood

Cyril of Jerusalem
"Do not, therefore, regard the bread and wine as simply that; for they are, according to the Master’s declaration, the body and blood of Christ. Even though the senses suggest to you the other, let faith make you firm. Do not judge in this matter by taste, but be fully assured by the faith, not doubting that you have been deemed worthy of the body and blood of Christ. . . . [Since you are] fully convinced that the apparent bread is not bread, even though it is sensible to the taste, but the body of Christ, and that the apparent wine is not wine, even though the taste would have it so, . . . partake of that bread as something spiritual, and put a cheerful face on your soul" (ibid., 22:6, 9).

Augustine
"What you see is the bread and the chalice; that is what your own eyes report to you. But what your faith obliges you to accept is that the bread is the body of Christ and the chalice is the blood of Christ. This has been said very briefly, which may perhaps be sufficient for faith; yet faith does not desire instruction" (ibid., 272).