In the Vedas this sin is called "nama bale papa buddhi" - knowingly commiting sin on the strength of the holy Name of God. If someone thinks "I am saved because I take shelter of the all purifying Name of God, therefore I can sin any way I like and I'll be cleansed", they are NOT purified even by the all-purfying Name of God, and thus pave their way to hellish life after death.
The main difference between Vedic religion (the original source of Hinduism) and Christianity is that hell is not permanent. It may be similiar to the Catholic description of Purgatory, except the Vedic descriptions of hells (there are many) are very severely terrible. But after a long, long time the soul is given another chance.
Jesus says, "Not everyone who says to me 'Lord, Lord,' shall enter the kingdom of heaven."
The difference is, Jesus claims to be the Gatekeeper.