When the Ghost of Christmas Past shows Scrooge a meeting he had as a young man with his fiancee, Belle, she is in the process of breaking off their engagement. She says that "another idol has displaced me," and elaborates, "a golden one."
This statement by Belle can be viewed as several different literary devices. First, the statement is hyperbole, an exaggeration for effect. When Belle says Scrooge used to consider her as his idol, she doesn't mean that he literally worshiped her, nor does she mean that he literally worships--that is, bows down and prays to--wealth and "Gain." As she notes a few lines later, "the master-passion, Gain, engrosses you." This is the literal, not exaggerated, way of saying that gold is Scrooge's idol.
One could also consider the "golden idol" reference as a symbol or metaphor. It compares Scrooge's devotion to making money to a religious devotee's worship of a sacred statue or icon. The idol symbolizes or represents the thing Scrooge has set his primary affections on, namely money.
Finally, there is definite religious imagery associated with the term "golden idol." In the Old Testament, the Israelites were charged in the Ten Commandments not to make any "graven image," or idol, yet they melted down their jewelry to form a golden calf to worship while Moses was on the mountain top receiving revelation from God. In the New Testament, Jesus updated the commandment for the Christian era by stating, "You cannot serve God and money." Scrooge has allowed his love of money to overtake his heart, and that has changed him from the good man he once was.
This simple statement by Belle can be seen as hyperbole, metaphor, symbol, and religious imagery.