When losing hope in Allah's mercy in considered as disbelief?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

Despairing of Allah's mercy in the sense that it removes one from Islam, is when a person believes that Allah is unable to forgive him — this is disbelief (kufr), as Allah the Exalted Said (what means): {Indeed, none despairs of relief from Allah except the disbelieving people} [Quran12:87]

At-Tahawi said in his creed (Al-'Aqidah At-Tahawiyyah): "Feeling secure [from Allah's punishment] and despairing [of His mercy] both remove one from the religion of Islam."

Its commentator, At-Turkistani, said: "As for their statement, 'Feeling secure and despairing both remove one from the religion' — this is because Allah the Exalted has both promised [reward] and threatened [punishment], and He is capable of both. So, feeling secure from what He has threatened implies believing Him incapable of punishing, and despairing of His mercy implies believing Him incapable of pardoning and forgiving — and each of these two removes one from the religion." [End of quote]

Allah Knows best