He thought of becoming a Christian; did he commit apostasy?
Question

I have been living in Britain for four years, and a few months ago I decided to become a Christian! But I did not do it. When I made that decision, I was saying to myself: If any of them asks me, what is your religion, I will say that I am a Christian and that Jesus is the “son of God” (I ask Allah to forgive me for that). But I also used to say to myself: No; Islam is the only true religion. 
Then one day I heard an exhortation of yours on YouTube, and it moved me deeply. I cancelled that intention that I had made to become Christian; in other words, I did not go to the church and did not do anything of that nature. 
My question is: are the words that I said to myself sufficient to have made me become a Christian? And therefore do I have to go to a mosque and utter the twin declaration of faith (shahaadatayn)?
Praise be to Allah.We praise Allah, may He be exalted, for having opened your eyes to the seriousness of what you did and your decision to turn to a religion other than Islam, and for His blessing you so that you changed your mind before it was too late. We would like to point out two things to you here: 
Firstly: 
What you did of deciding to become a disbeliever, and making your mind up to become a Christian constitutes apostasy from the religion of Islam, even if you did not actually enter the church or do any of the other actions that Christians do. Merely deciding to disbelieve constitutes disbelief in Allah, may He be exalted. The same applies to a person’s indecision as to whether he should turn to another religion or remain in Islam. 
Imam an-Nawawi (may Allah have mercy on him) said: Deciding to become a disbeliever in the future constitutes disbelief in the present. The same applies to indecision as to whether to disbelieve or not; it constitutes disbelief in the present. That also applies to making disbelief conditional upon some future matter.
End quote from Rawdat at-Taalibeen, 10/65. Something similar was said in Nihaayat al-Muhtaaj and elsewhere. See also: Majma‘ al-Anhaar (a Hanafi book), 1/688 
In Haashiyat al-Bujayrimi ‘ala al-Khateeb it says: 
Intending to disbelieve now or to disbelieve in the future constitutes disbelief in the present, because intending to be a Muslim forever is essential, so if he decides to disbelieve (in the future), he has already disbelieved. 
In Haashiyat al-Jamal (5/122) it says something similar. 
Secondly: 
It is not one of the conditions of your repentance that you should announce that in the mosque or Islamic centre, especially since you made that decision in your own mind and it did not result in spreading disbelief or mischief among the Muslims. As that is the case, it is sufficient for you to repent sincerely to your Lord, may He be glorified and exalted, to utter the Shahaadatayn, and adhere to the religion of Allah, may He be exalted, in all your affairs, and to disbelieve in all other religions. 
If you do ghusl before that, that would be good, in sha Allah. 
We advise you to seek knowledge and devote yourself to acts of worship and obedience, because the one who does that will increase and strengthen his faith. You should also be keen to offer supplication (du‘aa’), for it is the weapon of the believer, by means of which Allah will ward off evil from you and will protect you, by virtue of your remembrance of Him, from the tricks of your enemy and His enemy. 
And Allah knows best.