What is the ruling on someone ignorant of what is necessarily known of the religion, before he comes to know it? And is he obligated to pronounce the two testimonies if he comes to know it?

Something being unknown to some people makes it not necessarily known as a matter of religion, so the ignorant of it does not become an infidel. If it is necessarily known as a matter of religion, and someone is ignorant of it due to an excuse, such as being a recent convert to Islam or growing up in a remote desert, then they are excused. The information for which the ignorant person is not excused is a relative matter that varies with time and place. Whoever falls short in seeking obligatory knowledge is sinful, but does not become an infidel if they are ignorant of what they fell short in learning. An ignorant person who later learns a ruling is not required to pronounce the two testimonies of faith, because he has not exited Islam.