An interesting question. The Holy Books of all religions are full of sins that we can perpetrate against God. The Ten Commandments are a prime example and are pointed to as the rules God handed down to us directly.
While I will not argue with the sins listed in the Holy Books, even though many are archaic and meant for another time and place. I personally do not see the one sin that I consider the 'worse sin against God'.
The worse sin against God is 'claiming to speak for God'. There are a growing number of individuals that preach, in their respective religious affiliations, who not only preach the word of God, according to the Holy Books, but claim to be speaking for God. Their claims go well beyond the normal 'interpretation of scripture' and extend to actual conversations with God that they claim to be 'first person encounters'.
NO ONE SPEAKS FOR GOD!!! If the Holy Books are to be believed; God has laid down His laws and all we have to do is follow them to achieve a place with Him. Ministers who claim these first person encounters are stepping well beyond the duties that I prescribe to a minister. Which chiefly are the education of the masses to the Holy Books and guidance for those who need it.
I look at any of these claims of first person knowledge of God as dubious. Especially those that claim near daily conversations.
God is not a 'speaker' of wisdom that He rains down in us like a spring storm. God is a 'listener' who we can 'spill the beans to' and through reflection gain insight into our problems. Through the 'free will' that God has endowed all of us we can use this insight to solve our problems and move on with our lives.
Be very suspicious of anyone who claims that God has told them personally to do something. Especially if that something comes any where near your wallet. God does not ask for us to build cathedrals or monuments in His name. What he does ask is that we, each in his/her own way, achieve faith in Him. Through this faith you will achieve Gods graces. Not through listening to the 'usurpers of Gods word'!
I have noticed that my writings, over the past year, have increasingly taken on a religious theme. Kiddies I have not become a religious fanatic. What I have always truly enjoyed is the study of the Human Species. There is nothing more interesting than human/God interactions except maybe human/God/science interaction.
I am not an expert in this area but I am an eternal student of the subject. My observations are mine alone. I do not want or claim to seek any followers of my thoughts and that reason is very simple.
I believe that mine and your relationship with God is an individual relationship absent of any organized attempt to pigeon hole us into one belief or another. Follow your own path. Think for your self. Weigh all the information you garner. Make your own decisions. Form your own relationship with God according to your conclusions not that of any other person.