... This question is seen in many forms today and seems to be asked quite a bit. There are several directions to approach answering this question. My favorite approach is to go to the teaching of Jesus and to reinforce that with the explanations of Paul. First it is important to lay a foundation before we start to build. For the foundation we need salvation to stand on. Found in the Roman Road Method of leading folks to the foot of the cross is the passage found in Romans 10, verses 9 ad 10. Salvation requires faith to believe that Jesus is the Messiah and has the power to snatch us away from the clutches of Satan. Then we can call out to God for our salvation and the He will lead us into repentance.
... But, is salvation all there is to it? No, salvation is Fire Insurance, nothing more unless one chooses to follow God. There is an interesting and illuminating passage found in the teaching method of Jesus in Matthew 22. Jesus, as was His custom, taught in a parable. In this parable we find Jesus teaching of the celebration of the wedding feast that will occur in Heaven after the Bride, the members of the Church that chose to live a separated life to and for God, are raptured. Of course we find the Groom, Jesus, there and the members of the Bride but just as is illustrated in the parable, there are the guests.
... The guests are a seldom taught class of people in the church universal because that is not the calling of God but is rather a default position for a saved man. My preferred term for these folks is Pew Whale. Now it is very important that we all speak the same language and the term Pew Whale has a lot of usage and, in this case, needs defining. A Pew Whale, in general, is a person that comes in, sits down and wants to be fed. They do not study the Word of God for themselves and they do not pursue the fulfillment of the Great Commission. These are, by my understanding, the guests Jesus speaks of in the parable. They have their Fire Insurance but they have not obeyed, sought to follow God, His commandments to the best of their ability.
... The Bride will have members such as Peter, John, Paul and many others of worthy note. Then there is the absolutely amazing truth that people like myself will be members of the Bride. We do not have spotless testimonies and there are periods of great shame and failure in our life stories but because we repented of our past and because of the mercy and the grace of God we have been molded (Jer. 18) into New Creations. As new creatures in Christ we have sought after the separated life and lend ourselves to what ever the LORD may lead us to do.
... This same question is sometimes see as what church or domination is the Bride? There are many churches today that claim to be the Bride and that you must be a member of their sect to obtain entrance into the Kingdom of God but the study of scripture, all of scripture, demonstrates a completely different truth. Jesus never laid out a form for worship to be followed. He did state to the Samaritan Woman at the well that we must worship in Spirit and in Truth. It is my firm belief that there will be members of the Bride found in most churches that teach the Bible in it's true form. I even believe that a few members will be found in the LDS and JW congregations.
... Now... that I have lit the match that destroys, allow me to blow it out. These people are not saved in these heretical sects, they were saved before they were drawn away. I have a relative that is a case example. My aunt was a good Christian, taught in a good Christian Church in the country. Then she married and was influenced by her husband to join the Jehovah's Witness'. After a few years she came to her senses, went back to a Christian church and as a result ended up being divorced. She did not need to regain her salvation, salvation is eternal and that eternity begins the moment we are saved, she just needed to begin living in the truth once more. Had the Rapture occurred while she was in that congregation of heresy she would have been take up but would have had that mistake to be rekoned with. I suspect she might have lost a crown or two to that error.