Is merely thinking about converting to Christianity and uttering phrases like "Jesus is the Son of God" sufficient for conversion, and does this necessitate uttering the Two Testimonies again?

What happened, the resolve to convert to Christianity, is apostasy from Islam, even if you did not enter the church. For the resolve to disbelieve is disbelief, and similar to it is hesitation about converting to another religion. Imam Al-Nawawi said: "The resolve to disbelieve in the future is disbelief at present, and likewise, hesitation as to whether one disbelieves or not, is disbelief at present."

It is not a condition for repentance to announce it in the mosque. Rather, sincere repentance to Allah, uttering the two testimonies (Shahadatayn), and adhering to Islam are sufficient. It is recommended to perform ghusl (definition) (ritual bath) before that. It is advised to seek knowledge, engage in acts of obedience, and supplicate to strengthen faith.