Good Morning Peeler,
Back in Jesus day, the Pharisees and Sadducees were going by the laws of the OT, but they did not really abide by them. They twisted the laws of God. They made up their own laws because of the corruption inside of their hearts and justified their "righteousness" by saying it was God's will. They would make outward gestures and appearances of what they considered to be important and holy for the sole reason of impressing others. Because, Jesus knew their hearts were far from the love of God, he told them so and firmly held them accountable for what they were doing. They held jealousy and even revenge against Jesus and plotted to kill him because they felt Jesus was going to take over.
Everything has to do with what is inside of our hearts. What are we carrying inside of our hearts? The overflow of our hearts always comes out of our mouths. The Bible talks about how our own righteousness is like filthy rags in God's sight. True believers realize this and work within the Body of Christ doing God's will because God first loved us before we even knew Him. How humbling is that to think about? There is not one thing that we can do to earn our way into the Kingdom of God because God extended to us His gift of grace [unmerited favor]. The only way we can enter glory is through accepting and believing that God's son who was sacrificed upon that cross so we could have forgiveness of our sins. When we make that first step towards repentance in faith, The Holy Spirit will take you the rest of the way. Any other belief systems that do not regard Jesus as God's son is hearsay and false. "Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me." John 14:6
You further asked "Does mankind really know God?"
Yes. My own life is a testimony of the greatness of God's love and how he delivered me from the grip of darkness. Everyone is searching for love, but few realize it is God's love they are searching for and instead depend upon man's love or "try" other belief systems and they end up realizing they are only a bandaid compared to the fierce and faithful goodness of God's love that is offered through Jesus Christ. If you are searching Peeler. Search no more and humble yourself, read the book of John for yourself and listen to what the Holy Spirit is saying in your life. Have a great day!