The answer is: The heart

Matt 15:19; Mark 7:21

Ah, the intriguing question of the origin of sin! It's quite profound to consider the heart as the main culprit. In Matthew 15:19 and Mark 7:21, we find Jesus explaining that it’s not what goes into a person that defiles them but what comes out. It’s like that popular saying, “What’s in the heart comes out in the mouth!” Here, the heart is more than just the organ that pumps blood; it's a symbol for our thoughts, intentions, and feelings.

Think of the heart as the command center for our moral choices. When Jesus referred to the heart, He highlighted that our actions stem not from external influences but from within us. It opens up the idea that the struggle against sin is very much an internal battle.

Here's a fun point: In the ancient Hebrew mindset, the heart (or "leb" in Hebrew) encapsulates much more than just feelings—it means one's entire inner self, including thoughts, will, and affections. Additionally, it’s interesting to note that many biblical texts echo this concept. Proverbs 4:23 tells us to guard our hearts, for everything we do flows from it.

The notion of self-accountability invites each of us to reflect better upon our thoughts and motivations. Instead of blaming the outside world, it empowers us to take charge of our actions, recognizing that internal purity leads to genuine expressions of goodness. What a fascinating exploration, championing self-awareness alongside responsibility!