Explain verse 5:48

> And We have revealed to you, [O Muhammad], the Book [i.e., the Quran] > in truth, confirming that which preceded it of the Scripture and as a > criterion over it. So judge between them by what Allah has revealed > and do not follow their inclinations away from what has come to you of > the truth. To each of you We prescribed a law and a method. Had Allah > willed, He would have made you one nation [united in religion], but > [He intended] to test you in what He has given you; so race to [all > that is] good. To Allah is your return all together, and He will > [then] inform you concerning that over which you used to differ. > (Al-Ma'idah : 48) It says here that Jews, Christians and Muslims have their own way and law set for each to test them in what Allah has given them. Does this mean that each of them should stick to their own law? I am also convinced of this by the call "so race to good". Please explain. And more: what does "that over which you used to differ" mean? After all, the Torah and the Injil are also from Allah, and they do not differ from the Koran. And the past true Jews and Christians also did not disagree with Islam.