In Genesis 6:16 we learn that there was only one door: “…and set the door of the ark in its side.” I’ve been on many Navy ships and can tell you they are full 
of doors: fore and aft, amidships, athwart ship, and on every deck. The Ark was constructed with three decks, yet it appears that the ark only had a door on one deck. I’d guess it was the middle deck. Air craft carriers have their main door on the deck below the flight deck and when you see it, it’s perfectly obvious that that’s the perfect place for it. Now, getting back to the ark and Noah, There were two groups of people during the time of the flood – those who survived and those who didn’t. The ark was the only means of salvation. Those who survived walk into the ark through the one door.
One author writes, “Although it isn’t a popular teaching there are two types of people in God’s eyes – those who will be judged and those who are forgiven. Jesus is the only means of salvation and the only door into heaven. ‘I am the door. If anyone enters by Me, he will be saved.’ (John 10:9) Is there another way to heaven? The Bible answers clearly: ‘Jesus said to him, I am the way, the truth, and the life. No one comes to the Father except through Me.’ (John 14:6) ‘There is no other name under heaven given among men by which we must be saved.’ (Acts 4:12).”
Another observation about the one door is that it was secured by God Himself. Genesis 7:16 says, “And those that entered, male and female of all flesh, went in as God had commanded him. And the LORD shut him in.” There are two things we must know about this closed door. Once we come into “Christ” by faith as Noah came into the ark, God secures our salvation. As the “door” of the sheepfold, Jesus protects His sheep. When the sheep in Jesus’ day entered into the sheepfold, there was only one way in, but it usually didn’t have a door. The Shepherd would lie across the opening. No one could in or out except over His body. We are safe in Christ! Secondly, once God closes the door judgment fall on all those outside. This was what Christ meant in Luke 13:25: “When once the master of the house has risen and shut the door, and you begin to stand outside and to knock at the door, saying, ‘Lord, open to us,’ then he will answer you, ‘I do not know where you come from.’” Don’t be like the older brother of the prodigal son, who refused to go in and celebrate his brother’s return. Luke 15:28 says, “But he was angry and refused to go in. His father came out and entreated him…” I hear that as “please come in, please come in…” We’re safe and secure from all harms!