What is the ruling on a Muslim who said to a Christian that he (the Muslim) is "more Christian than him," intending to quote from their distorted book to expose contradictions and invite them to Islam? Is such a statement considered kufr (disbelief)?

Debating the People of the Book should only be undertaken by those who are firmly grounded in religious knowledge and confident in their ability to establish the argument against them. Narrating disbelief is not disbelief, and reading corrupted pages to demonstrate their corruption is permissible. His statement, "a Christian more than them," means that he is the true follower of Christ, who prophesied about Muhammad (peace be upon him), and he believes that Christ is a word from God and a spirit from Him, not a god. There is no harm in this statement as long as its intent is clear, and it is advisable to abandon obsessive thoughts.