The Rapture is not a guarantee for those who are saved but Heaven is. Many who have been saved by grace through faith in Christ Jesus during this Church Age are now in Heaven. The Rapture is an event which takes place at the end of the Church Age at which time Christ comes for His Bride. A bride as we know it is the one who is chosen above all others and for specific reasons. Not everyone at the wedding qualifies as The bride but yet they will be in attendance. You and I choose only one person as a bride and would not choose her if she were immature, unfaithful, unwilling and Not Ready and neither would Jesus. There are many saved in the Church today who are babies, living sinful lives and unwilling to repent - they are not Ready for The Groom's Return for them. Therefore, it will overtake them as a thief in the night and they will suffer great loss. When we are "Redeemed" by Christ we are made spotless and no sin can condemn us any longer. However, we are to keep ourselves unspotted from the world and repent continually from sins which separate us from close  "fellowship" with God -  "knowing" Him. (A term used for close intimate relationship between a man and woman.)
So as Jesus looks for His Bride He is looking for those who are "worthy" of Him - not babes, not unfaithful or lazy servants  but those who have chosen to follow Him and be His disciples. Paul pressed on towards the High calling in Christ Jesus and so should all of us. Jesus is the one who chooses His Bride out from among the many who are the called. It's His privilege and not to be assumed by anyone. Remember the parable of the banquet and what happens to those who assume the seat of honor next to Him. We who are His Body are all accountable to Him for obedience and performance - read Revelation 2,3. The Bride will be chosen out from the Body of Christ just as Eve was taken out from the body of Adam. Jesus being the second Adam. Those believers who are not ready for the Rapture event - when the Bride is taken out will not loose their salvation but will then be tested by the fires of Great Tribulation "purified" and made ready to meet their Lord.
Teaching that encourages all who "saved" in the Church are the Bride and will be raptured away guaranteed - ready or not.. is harmful - because it does not clearly warn those who have left their first love, the unrepentant, the lazy and unfaithful - to get ready now, be filled with The Spirit and watch because we do not know the day or the hour that he will come for any of us whether by death or rapture. The Good is news is-  all those saved by Christ will enter Heaven and Christ will choose His Bride and there is still time to Press On toward the "Mark" of His "Highest" calling in Christ Jesus.