وَعَدَ اللَّهُ الْمُؤْمِنِينَ وَالْمُؤْمِنَاتِ جَنَّاتٍ
Allah has promised believing men and women gardens. (Al-Tawbah 9:72)
The promise of Paradise to believers does not entail that every sinful believer enters it immediately without punishment. One may be pardoned, or undergo a limited punishment and then be released.
The point is that a person who retains faith is not forever excluded from the essential promise made to believers. It does not mean that all warnings against major sins become meaningless.