Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear sister in Islam, it gives us pleasure to receive your question and to see how Muslim converts are interested in knowing the teachings of Islam, which Allah has chosen for His servants as a way of life. According to the Qur’an, a true Muslim should refer to scholars to become well acquainted with the sound image of Islam.
We congratulate you on becoming a Muslim, and we hope you understand the tolerant and latitudinarian teachings of Islam which Allah revealed to guide mankind.
Answering your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states,
Islam teaches us that when a person embraces Islam all of his past sins are blotted out, and as such, he starts with a clean slate. While his past sins are completely forgiven, all of his good works, however, will be carried over to Islam, and thus, he will be fully rewarded for them. This clearly shows Allah’s infinite mercy and generosity towards new Muslims.
The Prophet (peace and blessings be on him) said, “Islam wipes out all the past sins of a person.” Once a person asked the Prophet (peace and blessings be on him), “In my past life (as a non-Muslim) I was guilty of many a trespasses and acts of lewdness; would I be forgiven?” He said, “Didn’t you know that Islam wipes out the past?”
Still on another occasion, one of the men asked the Prophet (peace and blessings be on him), “What about the good works I did in the past such as charities, freeing of slaves, feeding of the poor, etc?” The Prophet (peace and blessings be on him) replied, “You carry with you to Islam all of the good works you did before.”
Now let us come to the core issue: Should a person disclose his/her past sins to the spouse? The answer is definitely no. Since Allah has covered a person’s sins, it is not anyone’s business to seek to lift Allah’s cover of confidentiality. Thus it is unbecoming of anyone to probe into someone’s sinful past, especially one’s life before Islam.
Islam teaches us that no matter how ugly is a person’s past before Islam, it must never be used against him. If anyone does that, he is forgetting the fact that a new Muslim is like a newborn. It is also worth remembering that most of the great Companions of the Prophet (peace and blessings be on him), such as Caliph `Umar and others, had highly sinful pasts, and yet, no one found faults with them for the same.
After having said this, however, one must make a definite exception to the above rule. If someone is suffering from contagious diseases (such as being HIV-positive or afflicted with AIDS or other sexually transmitted diseases, etc.), it is incumbent upon him/her to disclose it to the prospective spouse [without adding details of one’s past sex life]. Failure to do so is a most heinous offense, as it may most likely expose the prospective spouse to unnecessary harm, which is not tolerated in Islam.
Allah Almighty knows best.
Excerpted, with slight modifications, from: www.islam.ca