Othello is of African descent, and Italian is his second or third language. He is also a convert to Christianity. How does each of these things affect how Othello is perceived by the other characters in Othello?

Shakespeare's Othello is set in an era of city-states; unlike in modern Italy, there is no real sense of a unified country. Indeed, the Venetians in the play have far more loyalty to their own city-state of Venice than to anything else, to the extent that even other Italians seem foreign to them. Iago notes that it is a particular affront that he has been passed over for Cassio because Cassio is "a Florentine"—essentially a foreigner. This being the case, Othello, who is a Moor, is facing a very difficult situation indeed.
That English is not his native language, yet he is able to tell captivating stories that can win over the beautiful Desdemona, says much about his eloquence and his character. Desdemona loves Othello's stories of foreign lands, men who carried their heads beneath their shoulders, and other such fantastical things. To her, Othello's otherness is intriguing; to those who have appointed him above white Venetians in the military, his eloquence despite this otherness also places him in high regard. Throughout the play, however, we are constantly reminded that Othello's blackness marks him as more different than any other foreigner.
Racial epithets pepper the play. Othello is "the thick lips," "the Moor," and it is particularly distasteful to Brabantio to hear that "an old black ram is tupping your white ewe." Iago knows that he is not the only racist person in Venice and uses this to his advantage. Othello is characterized as "lusty" and almost animalistic. Iago hates him because he fears that he has slept with his wife, supposedly, although there is really no evidence for this at all. Othello is looked upon with such suspicion that it is easy for Iago to turn others against him—arguably far easier than it would have been if Othello were white and Venetian. Othello has achieved much and fought hard for it, but in the end, he has to cling far harder to what he has than anyone else does. This is partly why Iago is so easily able to destroy him.

Othello's non-European background can be said to result in perceptions of him by the other characters that are paradoxically both negative and positive. Desdemona's love for him, as she states herself, is partly grounded in the sympathy she feels for him as an outsider, one who is held to a different standard than others and who has to "prove himself" to an extent that would not be required of a white person.
Iago's jealousy of Othello and his desire to destroy him are perhaps grounded in Iago's racism, of which he makes no secret to Roderigo. Iago uses Othello's outsider status to exploit his insecurity about his marriage to Desdemona and to convince Othello, with manufactured evidence, that Desdemona is unfaithful to him.
The Venetian authorities regard Othello perhaps more highly than they would "one of their own" precisely because he is a foreigner and has chosen to give his loyalty and his tremendous military skills to them, rather than having been born into their "tribe." The point about his not being a native speaker of their language is important. Othello's eloquence is a sign of his extraordinary brilliance and the nobility of his character. Even in his opening statement, when he confronts the brawl instigated by Iago, his words ("Keep up your bright swords, for the dew will rust them") reveal his calm in the face of danger and his ability to use language (like all Shakespeare's heroes) to convey a thought with such force that everyone is bowled over by it. At the same time, it's obvious that in spite of Othello's having every positive quality one could wish for in a son-in-law, Desdemona's father is prejudiced against Othello because of his race. So the deck is stacked against Othello from the beginning even with the Venetians who, unlike Iago, do not have any evil intentions.
The catastrophe at the end is almost unbearably tragic because Othello, in his last breath, still asserts his loyalty to the Europeans in the famous verse that begins "that in Aleppo once." And finally, Ludovico speaks for all the Venetians in his outrage over what has occurred: "Look on the tragic loading of this bed!" In some sense, it has required a disaster to change their minds and make them realize that not merely Iago but the whole "system" upon which their society is built has victimized and destroyed both Othello and Desdemona.