have worded their explanations of this in a variety of ways, but the gist of what they say is largely the same. Here is what Ibn Kathir has written:
The sound heart is one, which knows no impurity or idolatry
man’s good deeds, which provide the criterion for entrance into heaven, but rather his inward state. What will be of real value on Doomsday is the character a man has developed in this life. Those in possession