According to Wikipedia:
Christians (/ˈkrɪstʃən, -tiən/) are people who follow or adhere to Christianity, a monotheistic Abrahamic religion based on the life and teachings of Jesus Christ. The words Christ and Christian derive from the Koine Greek title Christós (Χριστός), a translation of the Biblical Hebrew term mashiach (מָשִׁיחַ) (usually rendered as messiah in English). While there are diverse interpretations of Christianity which sometimes conflict, they are united in believing that Jesus has a unique significance.
Paul does not refer to his followers as "Christian", but rather as "saints" (Romans 1:7) which means people set apart for God.
So why are people being called "Christians"?
Some quote 1 Peter 4:16 out of context to proof that it is a privilege to be called a "Christian" because it "glorifies" God.
However, if you read the prior verses too, it rather seems like it was an insult to Peter to be called a "Christian" and that he rather encouraged that his followers not to be ashamed for being called a "Christian" and that they should use the opportunity to "glorify God in that name".
Beloved, do not be surprised at the fiery trial when it comes upon you to test you, as though something strange were happening to you. But rejoice insofar as you share Christ’s sufferings, that you may also rejoice and be glad when his glory is revealed. If you are insulted for the name of Christ, you are blessed, because the Spirit of glory and of God rests upon you. But let none of you suffer as a murderer or a thief or an evildoer or fas a meddler. Yet if anyone suffers as a Christian, let him not be ashamed, but let him glorify God in that name. -- 1 Peter 4:12-16 (ESV)
According to Acts 11:26, the term "Christian" originated from Antioch:
And in Antioch the disciples were first called Christians. -- Acts 11:26 (ESV)
It does not say that they choose to be called "Christians" themselves. The only other recording in the Bible where someone was called a "Christian" is:
And as he was saying these things in his defense, Festus said with a loud voice, “Paul, you are out of your mind; your great learning is driving you out of your mind.”
But Paul said, “I am not out of my mind, most excellent Festus, but I am speaking true and rational words. For the king knows about these things, and to him I speak boldly. For I am persuaded that none of these things has escaped his notice, for this has not been done in a corner.”
“King Agrippa, do you believe the prophets? I know that you believe.”
And Agrippa said to Paul, “In a short time would you persuade me to be a Christian?”
-- Acts 26:24-28 (ESV)
Paul challenged Herod Agrippa II to belief. However, in response King Agrippa denied Paul's faith publicly, by mocking Paul in front of the council, to defend of his own reputation. King Agrippa did not honour Paul for being a Christian. It would rather make sense for King Agrippa to label Paul with something that would be offensive to him.
If it was a honour or an identity for Paul, to be called a "Christian" he would have referred to his followers as "Christians" and address them so, but he does not mention the word "Christian" even once in any of his letters, although he was very familiar with the name.
Another offensive name that was also used to describe Paul's followers was "Nazarenes":
And when he had been summoned, Tertullus began to accuse him, saying:
“Since through you we enjoy much peace, and since by your foresight, most excellent Felix, reforms are being made for this nation, in every way and everywhere we accept this with all gratitude. But, to detain you no further, I beg you in your kindness to hear us briefly. For we have found this man a plague, one who stirs up riots among all the Jews throughout the world and is a ringleader of the sect of the Nazarenes.”
-- Acts 24:2-5 (ESV)
According to a 2011 Pew Research Center survey, there were 2.2 billion Christians around the world in 2010. ... Christians make up the majority of the population in 158 countries and territories. 280 million Christians live as a minority. About half of all Christians worldwide are Catholic, while more than a third are Protestant (37%). Orthodox communions comprise 12% of the world's Christians. Other Christian groups make up the remainder.
This means, today various religions, church denominations, sects and even the Crusaders are all labeled under the same name "Christian" which could be very confusing. Depending on your audience, the "Christian" label could either be used to your benefit if the audience claim to be Christians, or could be a hindrance to people's when they hate "Christians" because of this misunderstanding.
There are no requirement anywhere in the Bible the one needs to converted to a "Christian" to be saved.