As-Salaamu ‘Alalikum wa Rahmatullahi wa Barakatuh,
Because you have not told me what exactly makes you fear the future, I am going to respond to your question assuming that you fear that his faith in Islam will not be sincere and that he only converted to marry you. Also, you may fear that your family may not trust his conversion and/or that his family may reject him if he converts.
If those are the reasons, then one way to figure out the answers to them is to take your time. After he takes his Shahada, wait—at least a few months—and see what happens before marrying him. During that time, you can see if he was sincere. You can also see what his family thinks and does to him.
If, on the other hand, you have to marry him immediately after he converts because you and he can’t wait (you fear you may commit zina), then at least be aware of the risk you are taking. You may end up suffering in your life with a person who is not a believer in Islam in his heart.
If he is a true believer in Islam in his heart, he will be able to deal with his family rejecting him—well, depending on how one defines the word “deal”. He may not like it if his family rejects him, but he will hold his ground if he is a sincere believer. But, if his conversion was only to marry you and you don’t marry him right away, then he may return to the religion of his family. It would be very important for you to find out the answer to this question before marrying him—beyond just what he tells you. If he takes his Shahada and deals with its fallout before you marry him, you will be able to be much more knowledgeable about what is really going on in his heart, In Sha’ Allah.
Remember, kindness is not belief. The Prophet’s uncle, Abu-Taalib, was all goodness to the Prophet (saw) but to the end he was an unbeliever. Abu-Safyan, on the other hand, fought the Prophet (saw) and became a Muslim in the end… May Allah (swt) be your guide in figuring out whether this person converts to Islam sincerely or to just be able to marry you.
Some people are nice out of pride. They are good because it makes them feel good about themselves. They are not conscious of or interested in doing good things to please Allah (swt). The problem with that sort of goodness (for personal pride) is that it is not reliable the way taqwa (God-consciousness) is.
Taqwa is love for Allah (swt) and fear of Allah (swt) that He (swt) will remove His protection of you in this life and punish you in Hell in the next. Love of Allah (who is Ar-Rahman and Ar-Raheem) gives us hope in His mercy that He (swt) will forgive our sins and save us from His punishment – which is removing His protection in this life and putting you in Hell Fire in the Next. This way of thinking is the most reliable because nothing causes fear of sinning like the Hell Fire, and nothing causes more devotion than love for Allah’s (swt) mercy. That love is the best! So, this guy may be a nice person now, but without taqwa that could change because the thing(s) behind his niceness are not those that would protect you securely.
I strongly suggest you let him take his Shahada and then see what he does after that. May Allah (swt) make it easy for you.
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