Bismillahi Arrahmani Arraheem
Assalaamo alaikum wa rahmatullahi wa barakatuhu
I firmly believe that Christianity is the number one rival and challenge of the true religion which is Islam. Christianity is a clever deception, it tries to portray itself as a typical monotheistic religion, preaching peace and good personal virtues, and worship of the one God. However, beneath this deceptive facade lie very evil and idolatrous doctrines, which the convert to christianity unfortunately becomes acquainted with after his conversion (or baptism). His missionaries and preachers will try to alleviate his anxiety by hammering in one point, like "Jesus is your personal saviour!", "he died on the cross for us for our sins, he was the perfect sacrifice, blah, blah, blah", but always at the back of the convert's mind he will have some anxiety as to the truth of christianity an will never be at peace and have full certainty that he is on the right path to salvation. You see, this is how Satan works, he likes to mix truth with falsehood, to cunningly deceive those who have less knowledge and whose faith is weak. But nothing short of the pure truth, which only Islam can claim to be, will be acceptable to Almighty Allah. The Seal of Prophets, Muhammad Mustafa (alaihi salaatu wa salaam) demonstrated this point by drawing a straight line in the sand, and then many horizontal lines on both sides of it, explaining that the straight line is the straight path of Islam, but Satan has placed many obstacles and crooked paths, trying to deceive man by many avenues and religions, of which the foremost is without a doubt Christianity.
The Problem with Christianity
The essential problem with Christianity is that it teaches idolatry, by giving divine qualities to a mere human being, Jesus (Alaihi salaam). It teaches Christians to worship a human being who could do nothing independently, but like us is entirely dependent himself on the grace of Allah. This is the greatest falsehood. Part of the reason the Holy Quran was revealed to mankind was to answer this deception of the false Christian doctrines and beliefs, such as worship of Jesus, believing that he died on the cross for our sins, the belief in trinity, and the practices of monastacism and mariolotry which is prevalent on a large segment of Christians. All of these objections and deviances have been so superbly answered by the Holy Quran, in Allah's own words. He has shown us the beauty of believing in the Oneness of Allah, which removes all anxiety and allows man to achieve his true potential as a slave and worshipper of Allah alone.
How to prove Christianity wrong
Christians claim several things about their Bible, the New Testament in general and the writings of the arch liar and anti christ Paul in particular, which is entirely false, being the apparent source for their evil doctrines. Some believe it is the infallible word of God, while others believe it is not error free, but nonetheless divinely inspired. The first class is the most easy to disprove, but the second class is more cunning and deceptive. However, if you can show them contradictions in their Bible with regard to doctrine, they will not be able to answer you. A mere glance at their so called holy books will reveal multitudes of flaws, inconsistensies, absurdities and scientific errors, and outright contradictions:
1. Jesus (alaihi salaam), whom they claim to be God, is not omnicient (Mark 13:32)
So how can God at any point in time not be completely All-Knowing, to ascribe ignorance to God is manifest falsehood
2. Is John the Baptist the Elijah that was to come again? Yes (Matthew 11:13-14), NO (John 1:21). So how can the so called "Gospels" be the infallible word of God, or else divinely inspired, when it is so outright contradictory, even on essential matters of belief, such as the status of John the Baptist, of whom all four Gospel writers seem to have made a great fuss over. No Christian to date has been able to satisfactory explain this big problem away with regard to John the Baptist being Elijah or not.
3. Jesus abolished the law with its commandments (Ephesians 2:15, Romans 7:6) This was the central focus and message of Paul, who in order to have the pagans convert to his new religion needed to compromise with their false and idolatrous worldview, and in order to free them of the "burden" of observing the law of God which would make them righteous, instead deceptively claimed that Jesus abolished the law with its commandments, and now instead belief alone (that too a false belief in Jesus's divinity) is sufficient for any Christian.
However, other parts of the New Testament seem to prove that Paul's gospel is not the gospel of Jesus Christ (Alaihi salaam). For example, Jesus explaining that he did NOT abolish the law (Matthew 5:17-18), and to further emphasize that point, taught his followers to strictly obey the Pharisees, the authorized teachers of the law, in EVERYTHING which they taught (Matthew 23:2-3) Obviously the Pharisees did not preach that Jesus abolished the law, or that faith alone is sufficient.
If every common Muslim was aware of these three simple things, the Christian missionaries would be hell bent on avoiding the doors of Muslim homes. I have seen these missionaries in action, they have no morals and dont act or teach honestly. They easily deceive ignorant Muslims. Satan thrives of ignorance, but the Truth of Knowledge will shatter his nefarious designs
Nay, We hurl the truth against falsehood, and it knocks out its brain, and behold, falsehood doth perish! (21:18)