All praise is due to Allah, peace and blessings be upon the Messenger of Allah and upon all his family and companions.
Allah Has Forbidden lying, cheating, and forgery for He Says: “O ye who believe! Fear God and be with those who are true (in word and deed).” {At-Tawbah/119}. Moreover, Prophet Mohammad (PBUH) said: “Should I inform you about the greatest of the great sins?" They said, "Yes, O Allah's Apostle!" He said, "To join others in worship with Allah and to be undutiful to one's parents." The Prophet then sat up after he had been reclining (on a pillow) and said: "And I warn you against giving a false witness, and he kept on saying that warning till we thought he would not stop.” {Bukhari}.
Consequently, it is forbidden that you carry on with your job which you have obtained by a forged certificate so as to acquit yourself before Allah. Sincere repentance requires that you be remorseful, seek forgiveness of Allah, and never do such a thing again. And Allah Knows Best.