If a person feels that both Islam and Christianity are good, how can he be sure that Islam is the truth and not Christianity?
This is a question that every human being has to ask himself and ultimately decide what to follow. When we look around us, we see people following all kinds of religions. Just as you say Christianity is good, I can also say that Buddhism and Hinduism are good. These are all religions, and all of them have commandments for good. If you look at the basic principles of all religions, they tell people to do good things and prohibit people from doing evil. So, it is not just Islam and Christianity, rather all the religions teach goodness. People attach themselves to them because religions have principles that are recognized to be good.
We all need to look at what we believe in and what others around us believe, and be sure within ourselves that what we are following is in fact the correct path. So, how do we know? The way in which we can determine this is actually quite simple if we consider that the purpose of our creation is the worship of God, which is something that we find in all religions. We now need to determine what the correct religion is and what in fact is the distorted version.
Islam teaches that Prophets were sent to every nation. Allah clearly states in the Qur'an:
"We have sent to every nation on earth a Messenger of God calling the people to worship God alone and to avoid the worship of false Gods." 
This is a basic belief that was shared by all the Prophets, that we should worship God alone and not worship false Gods. This is the criterion by which we may determine the true religion from the false. In the true religion, God alone is worshipped; whereas, in the false religion, God's creation is worshipped. The latter is the factor that is shared by all false and deviant religions which worship God through his creation. They believe they are worshipping God, but in fact they are worshipping God's creation. They have rationalized it in a variety of ways because the human mind is capable of giving all kinds of explanations and rationale, but the fact remains that they are worshipping God's creation.
Only in Islam will you find that God alone is worshipped and not any of His creations, as it is totally prohibited to pray to other than God. Praying to other than God is called shirk and it takes one outside the pale of Islam.
“Verily, Allah forgives not that partners should be set up with him (shirk), but He forgives (anything else) to whom He wills.” 
If we ask a Christian why he worships Jesus- a man- the Christian who has rationalized this will tell you that he is not worshipping Jesus the man but God who became this man. But the fact is that you are worshipping Jesus, the man. You've rationalized it to say that God became Jesus, the man, but it is still a man you are worshipping. This is why even in Christianity you have different sects and things that people argue about Jesus, like whether is he all God, part God, part man or which part of him was God and which part was man. It becomes a big problem for the philosophers of Christianity.
When we ask a Hindu or Buddhist why they worship a statue, the more intellectual of them will tell you that they are not worshipping the statue that you see, rather they are worshipping God, who is ever-present, everywhere, and who is present within the statue. The statue is a physical thing, we know we can break it and make another one. So here, rationale has been given to believe that they are worshipping God but the fact is that they are bowing down and performing acts of worship before a statue.
When you go through all the other religions you will find that the same principle exists. So if one wants to determine what is the true religion, as I said, it will be the religion wherein the One God alone is worshipped, and the false religion will be one in which God's creation is worshipped, all in the name of God, it will be worshipped in the name of God, but it will not be God.
This is our simple and basic principle which, if applied, we can determine what the true religion is. This is something that is applicable even to Muslims because as Islam spread to various places on earth, and people entered the fold of Islam, they brought with them some of their cultural practices and were influenced by the people around them. So we find that they have incorporated some principles that go against the fundamental principles of Islam. You may find in different parts of the Muslim world people who are praying to Muhammad, though this is prohibited in Islam.
“Muhammad (ﷺ) is no more than a Messenger, and indeed (many) Messengers have passed away before him...”
“Say (O Muhammad (ﷺ) ): "I am only a man like you. It has been inspired to me that your Ilah (God) is One Ilah (God i.e. Allah). So whoever hopes for the Meeting with his Lord, let him work righteousness and associate none as a partner in the worship of his Lord.” 
So, for example, if a person accepts Islam in an area, he may be taught to pray to prophet Muhammad (ﷺ). However, this is deviation, and is shirk, and is actually not Islam at all. We have to use this criterion to distinguish between those people who are in fact practicing Islam as it was revealed and those people who are practicing cultural traditions which have nothing to do with Islam.
 [Dr. B Philips, Purpose of creation, p.10]
 [Surah An-Nahl, 16:36]
 [Surah an-Nisaa, 4:48]
“… Hence, if Ash-shirk with its various types will not be forgiven, this necessitates that it should be feared with the utmost fear….” (Sheikh Salih Ibn Abdul ‘Aziz Ali Sheikh, The Destination of The Seeker of Truth, 32)
 [Dr. B Philips, The True Message of Jesus Christ, 62-69]
 [There are many verses in the Qur’an which show that Prophet Muhammad (ﷺ) was nothing but a Prophet. He was and is not worthy of worship, He was a man who was sent as Messenger to every human being, e.g :
(Surah Al-Imraan , 3:144)
(Surah Kahf , 15: 110)]< Back to Questions