Islam is inside everyone of us since he was a child. That is why when non-Muslims become Muslims people prefer to use the word revert and not convert to Islam. These are simply the reasons:
1- Inside everyone of us a feeling that there is one God who created us and the universe.
2- Inside everyone of us a meaning to life beyond the simple eat and reproduce like animals thinking.
3- The best of this meaning is a God living in us, with us and all around us.
4- With the feeling he is watching us, expecting us and caring for us we feel we are living by him and for him. A 24h and seven days a week relation.
5- We feel that we broke or weakened the relation when we disobey him. The evil has inner feeling inside us and our conciseness develop more when we have a close relation with God.
6- People who went far away particularly when they indulged in sins felt they are away from God.
7- When they come back to him they feel the bond again and they feel they ought to follow his path. People who rationalize sins for reasons untrue like you already saved or morals are relative feel something is lacking or feel emptiness.
8- When we worship God we feel satisfaction and inner serenity.
9- When we do not worship we feel isolated and feel emptiness inside us even if we have countless friends.
10- We feel the best moral code comes from God. Laws teach us fear and be slaves to the laws. When we do the right because we believe it is right and will make God happy with us we feel more motivated and happy doing the good.
11- God like we know in Islam does not require you to be an angel. You do not give up for life or distribute all your money on charities. He expects from you errors but he wants you to do the less of errors in count and gravity.
12- He enjoys when you thank him in good times or plead with him in bad times. When you are praising as a devout righteous person or when you coming back to him as a pleading sinner.
13- The reality of good and evil inside everyone of us is well appreciated in Islam and the struggle between the good and evil in us is called the great Jihad (great struggle).
For all of the above and many more Islam is inside us. Each if you read you will find in the Quran at least one verse talking about one of the above points. So people who find Islam or convert to Islam are in fact reverting to Islam. Read also in my blog Islam the religion of the common sense.