Assalam Alaykum, Dear Brother, how can we say that Islam is the only true religion when there are also other religions which speak about doing good and forbidding evil?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

What is meant by Islam being the true religion is that it abrogated all previous religions. After the advent of Islam, Allah will not accept a religion other than Islam.

However, this does not mean that other religions do not include some truth as the Quran confirmed the previous heavenly Revealed Books because all the revelations were from Allah, it is for this reason that the religion of all the Prophets is one, which is Islam.

Hence, there are some matters upon which all the heavenly religions agreed upon, like Tawheed [the Oneness of Allah], the prophecies and other matters of Faith. Also, all religions agreed upon encouraging the moral principles, like truthfulness, honesty and the like, and warned people against the worst evil matters, like lying, cheating and the like. Rather, enjoining what is good and forbidding what is evil is something upon which the rational people from various heavenly religions or other (man-made) religions agreed upon.

Hence, if we find a religion among these religions that enjoins good or forbids evil, then this does not mean that it is a true religion or that it is upon the truth.

Allah Knows best.