رَبَّنَا آمَنَّا فَاغْفِرْ لَنَا وَارْحَمْنَا وَأَنْتَ خَيْرُ الرَّاحِمِينَ.
Our Lord, we have believed, so forgive us and have mercy upon us, and You are the best of the merciful.
Rabbana amanna faghfir lana warhamna wa Anta khayrurrahimin.
Before asking Allah’s forgiveness and mercy, the Believers appeal to Him by means of their faith and their belief in Him, in His prophets, and in His books. Eman is the single greatest act of worship, and so it is appropriate that a Believer would use it to appeal to Allah. They end the supplication by appealing to Allah by means of His mercy and by acknowledging that He is the best and Most Merciful of those who are merciful.