Ah ha, you might notice what I just did. I used another word which is very similar to presumptuous: assumption. In fact, both words have the same root of "sume". As I looked their meanings up this morning I found out that the root word is actually the Latin word "sumere", which means "to put on or take up."
This became relevant to me this morning because of Psalm 19. The Psalmist David says this:
Who can discern his errors? Acquit me of hidden faults.There are two types of sin, and we need repentance for both. There's hidden sin--sin which we don't see but it's there. And there are presumptuous sins, and that's where I stumbled (no pun intended). What does it mean to sin presumptuously, I wondered?
Also keep back Your servant from presumptuous sins;
Let them not rule over me;
Then I will be blameless,
And I shall be acquitted of great transgression. (Psalm 19:12-13)
To sin means to trespass. It means to go beyond limits set by God. Why do we trespass? Well, we either trespass because we didn't know what God's good limit was, or we trespass because we thought His limit didn't apply to us. That is, we determined that we knew better than God. When we commit "presumptuous" sin we "presume" to know more than God about what we can do or handle.
Consider a hiker walking in a treacherous terrain. A ranger find him and says, "Sir, you need to turn around because this area is prone to rockslides. In fact, five people have died here already this year. Didn't you see the signs posted? For your own safety it's actually illegal to be here...you're trespassing."
The hiker responds honestly, "Oh, I'm sorry, I didn't know it was so dangerous. No, I didn't see any signs but I'll turn around right now."
Now consider a hiker who sees a sign that says "No Trespassing. Rockslides common ahead", but he continues on anyway. The hiker rationalizes that the sign doesn't apply to him because he clearly knows more about his ability to hike dangerous terrain than the people who printed the sign. "The sign was printed for stupid people or incapable people," he thinks, and ignores the warning.
The latter hiker trespassed presumptuously. He presumed to know more than the maker of the sign. He took the warning for granted. This is the second type of sin from which David wants protection when he says, "Also keep back Your servant from presumptuous sins; Let them not rule over me;"
I could stop here but I'm not sure just knowing the two types of sin is very helpful in pursuing holiness. What could prove more helpful is understanding that David actually asked God to hold him back from sinning presumptuously. So praying for God's grace to prevent sin in our lives is where we should start.