They were ignorant of God's righteousness. Righteousness simply means right doing; rectitude. According to Psalm 119:172, righteousness is defined as the commandments of God. That is right doing! So we see here that, by definition, these people did not know of God's right doing - God's right way, God's right method, God's commandments. We see that confirmed in Mark 7, where Jesus said:
Brethren, it says in Psalm 119:172 that ALL of God's commandments are righteousness. Understanding is the result of doing the commands of God. When we stop doing, we begin to lose the understanding. It is not something that will happen quickly. It will not happen overnight, or in the blink of the eye. But, in the biblical milieu there, You use it, or you lose it.