If after tawbah' a person intentionally repeats the major sin by saying that Allah is mercifull He will forgive..and keeps on doing like that , then what does Islam says?

A person engaging in sin with the perception of Allah?s forgiveness is falling into the trap of Shaytaan. Allah Ta?ala accepts the tawba of one on condition it is done with sincerity.

Repeating the sin displays one being insincere in his tawbah. You should always consider the fact that after having committed, death is imminent. It may take you at any time, what if death approached before having repented?

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai