When God gave the instructions for the building of the ark, He told Noah to put one door in the side of the ark: “the door of the ark shalt thou set in the side thereof” (Gen. 6:16).
The ark is a picture of Christ. Being in the ark, Noah and his family were safe from the judgment of the worldwide flood. Likewise, being in Christ, we are safe from the ultimate judgment of the lake of fire. He is the “Ark” that delivers us from the judgment of God for eternity.
The door of the ark is a type of Christ as well. He is the door by Whom we enter into salvation. As the Lord taught Israel in His earthly ministry, and is a truth for us in the Body of Christ too, “I am the door: by Me if any man enter in, he shall be saved…” (John 10:9).
The way of salvation in Noah’s day was the wooden ark with its single door, and the way of eternal salvation today is by the wooden cross upon which Christ died. And as the ark had only a single door, so it is that Christ is the one way, the only way to eternal safety. source