This of course is not so. The story came about after letters were published where the Pope was writing to an atheist columnist named Dr. Scalfari. In the letters, Dr. Scalfari asks the Pope a series of questions. The Pope replies to them briefly and in one of the questions, mentions the reliance of conscience. This is where the media and others jumped the gun. They interpreted this as if the Pope said that our conscience decides our salvation. Some Evangelicals fell for the media’s word play as well, see:
What the Pope was referring to in the response were mercy and forgiveness. If an atheist is good and in his/her last days asks for mercy or forgiveness, there is a possibility that he or she may go to heaven even if he/she never stepped foot in a church or ever prayed before this final conversion. See more here: Pope Francis Atheists Saved.
This is nothing new. We see this in the Gospel with Jesus and St. Dismas, or the “good thief.” (Luke 23:32-43) Here we have a man who was far from God and in his last days was right next to Him. He asked for forgiveness and Christ promised him that he would be in paradise.