Paul said to Elymas the magician: "You son of the devil, you enemy of all righteousness, full of all deceit and villainy, will you not stop making crooked the straight paths of the Lord?" (Acts 13:10 ESV)
Not everything in the Bible is difficult to understand.
There are those who know the truth, but simply refuse to comply with it; and therefore, they do whatever they can to make the simple truth seem hard to understand.
In the first century they were identified as Gnostics, and 1 John addresses it. There were many forms of Gnosticism, and quite honestly not all of the forms were anti-truth or anti-God; but most of them were because they sought to elevate man by means of himself rather than elevating man by means of God (see Colossians 2:20-23).
There's a difference between self-righteousness and true righteousness. Self-righteousness is based upon what "self" thinks, and can be unreliable; true righteousness is based upon the reliable standard of what God our Creator—whose ways and thoughts are higher than ours (Isaiah 55:8-9)—thinks.
Lead me, O LORD, in your righteousness
because of my enemies;
make your way straight before me.
For there is no truth in their mouth;
their inmost self is destruction;
their throat is an open grave;
they flatter with their tongue. (Psalm 5:8-9 ESV)
