How do I respond to a Christian if he asks me: Is he an infidel or not, and will my failure to respond make me an infidel?

When debating with an opponent, one should seek wisdom and good admonition, and use the best methods that are likely to have an impact. "And do not dispute with the People of the Scripture except with that which is best."

Directly telling someone they are a disbeliever might be off-putting to the one being invited to Islam. One can remain silent about it if there is no benefit in declaring it, and this is not considered disbelief or doubt in their disbelief.

The discussion can be redirected to another matter, such as telling them: "Whether you are a disbeliever or not is not our topic; rather, we should investigate which of us is on the truth so that we adhere to the truth," similar to the Qur'anic style: "Say, 'Who provides for you from the heavens and the earth?' Say, 'Allah.' And indeed, we or you are either on guidance or in clear error."