What warning does Digory find written on the golden gates outside the sacred garden in Chap. 13?  

The warning words are: Come in by the gold gates or not at all, Take of my fruit for others or forbear, For those who steal or those who climb my wall Shall find their heart's desire and find despair.

What is Digory tempted to do? Can you think of any parallels (similar situations) in the Bible?  

He tempted to eat the apple because he smelt it. and a terrible thirst and hunger came over him to push him to taste the fruit.

In the Bible, the serpent tempts Eve to eat the apple of the tree of life in the middle of the garden of Eden. The same thing is an apple and try to entice others to eat.

What helps Digory not to do the wrong thing?  

First I think it’s Digory’s mother who teach him to awfully strict about keeping promises, not stealing, which made him keep promise that brought the apple to the lion. When the Witch tempted Digory to take out the apple for his own world to cure his mother, he thought his mother would tell him not to do it.

Second it’s Polly that helps him no to do wrong things. When the Witch told Digory to leave the little girl in Narnia, this meanness of the suggestion made all the other things the Witch had been saying to him sound false and hollow. He cleared that what the Witch said was her trick.