This question is often asked when a tract is given or when we invite someone to a meeting for the study of the Word of God. Doubtless it is a wise question, especially in these days of so much confusion.
But what would have happened if you had asked the same question in the days of the apostles? Let's suppose that you lived at that time, and one day you met the Apostle Peter. You would have asked him, "Peter, what denomination is this?" Could you imagine the answer? Peter, doubtless, would have scratched his head, completely perplexed, because then there were no denominations. Man has tried to add them to the divine order, and has created his own systems.
God has a church in this world, but it isn't a church which you can join. You can very well become a member of a church made by men, and then leave it if it doesn't please you. But you can never become a member of the church of God, which is called the church of the living God (1 Tim. 3:15) and leave it at your will. He who is saved by believing in the Lord is automatically "joined."