“Build a large boat from cypress wood and waterproof it with tar, inside and out. Then construct decks and stalls throughout its interior." (NLT, emphasis mine)
Now this is not my first, second or even third time of reading this verse and yet, it "sounded" like I'd never heard or read it before. As I heard then re-read it, it was obvious that God needed the ark to be doubly protected from the water otherwise, it could sink. The Holy Spirit then explained that God expects the same thing of His children - you and I.
God tells us to protect our insides - our heart - from everything that could corrupt us and watch our external interactions from corruption too.
Proverbs 4:23 says "Be careful what you think [Above all that you guard, protect your heart], because your thoughts run your life [life flows from it]." (Expanded Bible)
Luke 6:45 takes that verse a bit further so we understand the warning -
Good people bring good things out of the good they stored [treasured] in their hearts. But evil people bring evil things out of the evil they stored [treasured] in their hearts. People speak the things that are in their hearts [For the mouth speaks what overflows from the heart]. (Expanded Bible)
Turning to the external (i.e. outside), 1 Corinthians 15:33 warns "Don’t be fooled by those who say such things, for “bad company corrupts good character." (NLT)
Psalm 24:4 says The one who has clean hands and a pure heart,
who does not trust in an idol or swear by a false god. (NIV)