Is today's Judaism a sin?

It is one of those questions you can answer yes or no depending on the context, because it is not black and white. In general, any religion that is not Christian falls short, but there are degrees; even among our Protestant brethren you have people who look Christian and live by Christian morals. The same goes for Judaism: there are things left over from it that may prompt a Jewish person to seek after God, and he may seek God his whole life without ever being convinced of the Catholic religion, so what God does with him is God's decision, not ours. The Church holds that God is the final judge. Catholics have a better chance of salvation because they can grow up in a Catholic family, receive the sacraments, and be put on the road, but the hard part is that you must persevere to the end and obey everything the Church teaches, and if you reject one of her doctrines you could be in mortal sin and lose your salvation. The people of the world do not have that hard part because they do not believe it in the first place. Our task is simply to preach the gospel to the Jewish person, tell him the Catholic Church is the way of salvation, and let the chips fall as they may.