Short answer
You keep more than you might expect: belief in one Creator, love for Jesus and Mary, the prophets and a life of prayer. What changes is belief about God and Jesus: no Trinity, Jesus as a prophet rather than God, and Muhammad ﷺ as the final messenger. You enter Islam by saying the shahada with belief. No baptism or ceremony is needed.
A Christian becomes Muslim by sincerely saying the shahada. Hanafi jurists add that a person from a faith that already affirms prophets should also disown beliefs contrary to Islam, such as the Trinity, which the shahada already implies. The duties of Islam begin from that moment, and missed worship from before Islam is not made up.
The four schools agree on this.
Explanation
A Christian who becomes Muslim does not leave Jesus behind. Muslims must believe in him as the Messiah, a great messenger, born of the virgin Mary, who worked miracles by Allah's permission and will return. The Prophet ﷺ said a person of the Book who believed in his own prophet and then in Muhammad ﷺ receives a double reward.
What changes:
- Belief about God: Allah is one, with no Trinity and no son.
- Belief about Jesus: a human prophet and servant of Allah, not crucified, and not a saviour who died for sins.
- Scripture: the Quran is the final word of Allah. Muslims believe the original Gospel was revealed to Jesus, but follow the Quran.
- Worship: five daily prayers, the fast of Ramadan, zakat, and Hajj if able.
- Repentance: directly to Allah, with no priest in between.
- Food and drink: no pork and no alcohol.
What does not change: your duty to your parents and family. A convert should stay kind and close to Christian parents and relatives, and nothing from before Islam needs to be confessed. See treating non-Muslim parents and telling your family.
To start, say the shahada, take a ghusl, and learn how to pray step by step. If you are married, read when one spouse converts.
The evidence
Quran, Surah Aal-i-Imraan 3:84
قُلْ ءَامَنَّا بِٱللَّهِ وَمَآ أُنزِلَ عَلَيْنَا وَمَآ أُنزِلَ عَلَىٰٓ إِبْرَٰهِيمَ وَإِسْمَٰعِيلَ وَإِسْحَٰقَ وَيَعْقُوبَ وَٱلْأَسْبَاطِ وَمَآ أُوتِىَ مُوسَىٰ وَعِيسَىٰ وَٱلنَّبِيُّونَ مِن رَّبِّهِمْ لَا نُفَرِّقُ بَيْنَ أَحَدٍۢ مِّنْهُمْ وَنَحْنُ لَهُۥ مُسْلِمُونَ ﴿٨٤﴾
Say, "We have believed in Allah and in what was revealed to us and what was revealed to Abraham, Ishmael, Isaac, Jacob, and the Descendants, and in what was given to Moses and Jesus and to the prophets from their Lord. We make no distinction between any of them, and we are Muslims [submitting] to Him."
A Muslim believes in Jesus and all the prophets, so conversion keeps that faith.
Quran, Surah Al-Maaida 5:83
وَإِذَا سَمِعُوا۟ مَآ أُنزِلَ إِلَى ٱلرَّسُولِ تَرَىٰٓ أَعْيُنَهُمْ تَفِيضُ مِنَ ٱلدَّمْعِ مِمَّا عَرَفُوا۟ مِنَ ٱلْحَقِّ ۖ يَقُولُونَ رَبَّنَآ ءَامَنَّا فَٱكْتُبْنَا مَعَ ٱلشَّٰهِدِينَ ﴿٨٣﴾
And when they hear what has been revealed to the Messenger, you see their eyes overflowing with tears because of what they have recognized of the truth. They say, "Our Lord, we have believed, so register us among the witnesses.
The Quran describes Christians moved to faith on hearing the revelation.
Sahih al-Bukhari 97
Sahih... ثلاثة لهم أجران رجل من أهل الكتاب آمن بنبيه وآمن بمحمد ...
"Three people receive a double reward, among them a man of the People of the Book who believed in his prophet and then believed in Muhammad."
A Christian or Jew who accepts Islam is promised a double reward.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Jihad, Bab al-Murtadd, the discussion of how a person of the Book becomes Muslim. Discusses disowning contrary beliefs alongside the shahada.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.