The logic of this statement implies that it is impossible for a Christian to know with certainty what God wants and that sin is not real, but only subjective.
In reality, 99% of Christians - Catholic, Orthodox, Anglican and Reformed - agreed on almost every last detail of this question until the 1930s.
Yes, it is impossible for a Christian to know with certainty what God wants. We live the way we live based on faith that we are understanding God's scripture and God's communications with us correctly. I will never claim to have figured it out, nor will I believe anyone who says they have.
"Sin is not real, but only subjective."
WRONG. Scripture is clear about what is sin and what is not.
Will all Christians and supposed Christians ever agree? Absolutely not.