All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
You are not sinful for saying this sentence, let alone having committed Kufr because of it. You do not have to explain to them that you are a Muslim.
You have done well by not taking those books and do not take them again, as it is impermissible to read books of misguided people except for a correct purpose, like a scholar who reads them in order to refute the falsehood in them and so on.
Jaabir narrated that ‘Umar came to the Prophet and said: “We hear narrations from the Jews, which we like, should we write some of them? He (the Prophet) said: "Are you in doubt, as were the Jews and the Christians? I have brought it (Islam) to you pure and white [i.e. free from any fault], and were Moses alive he would have followed me (without having any other alternative)." [Ahmad, and Al-Albaani classified it as Hasan]
There is no particular method to respond to those preachers; it is enough to turn away from them.
Allah knows best.