When Bassanio requests a loan from Antonio in the first scene of the play, he describes his unfortunate financial state—having spent the money already lent to him—and compares himself to an immature youngster:
I owe you [Antonio] much, and, like a willful youth,
That which I owe is lost.
This simile speaks to Bassanio's nature as either foolish or knowingly manipulative. Bassanio tries to describe himself as rash, having lost the money without knowing any better—but the use of the word "willful" suggests that Bassanio was deliberate in his actions. He also changes the subject of the sentence in the final clause: at first, Bassanio is the subject ("I owe you much"), but by the end, the money becomes the subject and the verb becomes passive ("That which I owe is lost"), suggesting that the money just disappeared and that Bassanio bears no responsibility in the matter. Bassanio is clearly either clueless about the truth of the situation or fully aware of what he has done and is actively trying to manipulate Antonio further. The play's use of the "willful youth" simile ultimately leaves this aspect of his character open to the audience's interpretation. Either way, Bassanio's words establish that he isn't a very responsible character.
When Shylock cites the Book of Genesis to support the practice of usury in Act 1, Scene 3, Antonio exclaims to Bassanio,
The devil can cite Scripture for his purpose!
An evil soul producing holy witness
Is like a villain with a smiling cheek,
A goodly apple rotten at the heart.
O, what a goodly outside falsehood hath!
In this simile, Antonio compares a malicious person who quotes the Bible to a villain who feigns righteousness, or an appealing-looking apple with a rotten core. While Antonio uses this simile to describe Shylock, the sentiment resonates far beyond him. In fact, the idea of a person who seems morally sound, but who is actually villainous, applies to Antonio himself, as well as his friends. Antonio is well-regarded in his community, yet abuses Shylock; Bassanio is beloved by Antonio, yet he manipulates the man who offers him his life; Portia preaches mercy and asserts that she does not judge people based on the color of their skin, yet she proves merciless and racist. Overall, the Christians in Merchant decide to see each other as having a "goodly outside," despite their inner cruelty—and though they define themselves by their religious beliefs, they often stray from their guiding principles.