God's patience has an end, and the Flood is a figure of baptism.
God waited in the days of Noah while the ark was being built. Eight souls were saved by water, and Peter calls that a figure of baptism.
1 Peter 3:20-21 (KJV): Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ:
God is still longsuffering today. He wants all to come to repentance.
2 Peter 3:9 (KJV): The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.
But there came a day when the LORD shut the door.
Genesis 7:16 (KJV): And they that went in, went in male and female of all flesh, as God had commanded him: and the LORD shut him in.
Today the way in is the same one Peter preached on the day of Pentecost: repent and be baptized in the name of Jesus Christ.
Acts 2:38 (KJV): Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.