If we receive infinite grace through bread and wine as the Catholic theology say, why does a person still sin against God? If we receive infinite grace through bread and wine as the Catholic theology say, why does a person still sin against God?

Because during transubstantiation doesn't infinite grace bestowed on each bread and wine?
If we receive infinite grace through bread and wine as the Catholic theology say, why does a person still sin against God?
Answer
While it is true that all faithfuls or members of the Catholic Church can avail of the infinite graces through participation in the Holy Mass and by receiving the Holy Communion.
Catholic Church also teaches that one must received the Holy Communion in a &quot;state of grace&quot;, meaning if one is aware that he/she had committed mortal sins or simply bothered by his/her conscience, the Church Sacraments of Confession must be availed first by the faithfuls before receiving Holy Communion.
Also, let us not forget that the &quot;Holy Eucharist&quot; is the means a faithful can be with intimate union with Christ by eating his &quot;Flesh &amp; Blood&quot; as Jesus teaches in John Gospel.
John 6:50-58 New International Version (NIV)

&quot;But here is the bread that comes down from heaven, which anyone may eat and not die. I am the living bread that came down from heaven. Whoever eats this bread will live forever. This bread is my flesh, which I will give for the life of the world.”
Then the Jews began to argue sharply among themselves, “How can this man give us his flesh to eat?”
Jesus said to them, “Very truly I tell you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you.  Whoever eats my flesh and drinks my blood has eternal life, and I will raise them up at the last day.  For my flesh is real food and my blood is real drink.  Whoever eats my flesh and drinks my blood remains in me, and I in them.  Just as the living Father sent me and I live because of the Father, so the one who feeds on me will live because of me. (John6:50-58)

Catholics eventhough received plenty or as you said infinite graces through the Holy Eucharist still has to persevere in his daily Christian Life.As St.Paul teaches in his letter to the Hebrews;
Hebrews 12:1-3 New International Version (NIV)

Therefore, since we are surrounded by such a great cloud of witnesses, let us throw off everything that hinders and the sin that so easily entangles. And let us run with perseverance the race marked out for us, fixing our eyes on Jesus, the pioneer and perfecter of faith. For the joy set before him he endured the cross, scorning its shame, and sat down at the right hand of the throne of God. Consider him who endured such opposition from sinners, so that you will not grow weary and lose heart.(Hebrews12:1-3)

St.Paul teaches us that we must not grow weary &amp; lose heart as our battle with the enemy of our salvation does not end until we expiate our last breathe.
And we also reminded by St.Peter with regards to the enemy of our salvation;
1 Peter 5:8-11 New International Version (NIV)

Be alert and of sober mind. Your enemy the devil prowls around like a roaring lion looking for someone to devour. Resist him, standing firm in the faith, because you know that the family of believers throughout the world is undergoing the same kind of sufferings.
And the God of all grace, who called you to his eternal glory in Christ, after you have suffered a little while, will himself restore you and make you strong, firm and steadfast. 11 To him be the power for ever and ever. Amen.(!Peter5:8-11)

With Apostles Peter &amp; Paul teaching Catholics who received the Holy Eucharist must therefore, always stay vigilant and sober because satan is just around the corner waiting for a perfect time for us to let down our guard.