No one who is begotten by God commits sin
Firstly, the word sin is not as broadly as some would use it today. Some seem so obsessed with sin that even acts of bad manners are classified as sin. I remember a Rabbi when I was growing up who someone chastised for saying a bad word at a football game. He proceeded to school this person in the entire list of words not forbidden by the Torah. Cussin' as we say in the south may be bad manners but unless you are taking the Lord's name in vain, there is nothing in the Bible that forbids it. This is not to say that it is recommended. I use that example to point our that when St. John says that no one who is begotten of God commits sin, he is using the word in the strict sense. The catechism of the Catholic Church defines a sin as "an utterance, a deed, or a desire contrary to the eternal law."(CCC 1849)
More than the definition of sin is how St. John describes our ability to live without sinning. He says that No one who is begotten of God commits sin, because God's seed remains in him. For us as Christians St. John reminds us that we believe that three sacraments imprint an "indelible character." When one is baptized, confirmed or ordained; "the Father has set his seal" on us in away that cannot be undone. Even when we sin, this indelible character cannot be erased. In that sense, we really have no excuse for our behavior. This seed remains lodged in us and speaks to us through our conscience enabling us to know, when we will listen.
In short, if we are begotten by God, sin takes effort. We must willfully choose to either ignore our conscience all together or hear it but act contrary to it. For those reborn by baptism, sin is not inevitable. As we walk through this day, may we let the seed that is planted in us bear good fruit.