Is it permissible for a person to commit sins, saying that God is Forgiving and will pardon all his sins, and that he will seek God's forgiveness later?

A Muslim must combine hope and fear. He should neither feel safe from Allah’s plan nor despair of His mercy, as Allah Almighty says: "Declare, O Muhammad, to My servants that truly I am the Oft-Forgiving, the Most Merciful. And that My punishment is indeed the most painful punishment." And "And who despairs of the mercy of his Lord except those astray?" And "Do not despair of the mercy of Allah. Indeed, none despairs of the mercy of Allah except the disbelieving people." And "Did they then feel secure from the plan of Allah? But no one feels secure from the plan of Allah except the losing people."

One must not procrastinate in repentance but rather hasten to it with the resolve not to return to sin, for Allah forgives everything but shirk (associating partners with Him) for whomever He wills. Persisting in sins darkens the heart and covers it with rust (Rān), as stated in the Hadith: "When a servant commits a sin, a black spot is made in his heart. If he repents, ceases, and seeks forgiveness, his heart is polished. But if he returns [to sin], it increases until it covers his heart. That is the 'Rān' which Allah Almighty mentioned: 'No! But on their hearts is the Rān (rust) which they used to earn.'"