Firstly, we have to recognize that you are asking, at bottom, the same question Pilate asks in his examination of Christ, in John 18. “What is truth?” Secondly, you cannot, as a friend often says, speak of truth without speaking of consistency. Thirdly, your system of beliefs (or the lack of a system, for that matter) will, and inevitably so, influence your consideration of truth claims.
What religion is true? To answer that, you have to have an answer for “What is truth?” Jesus tells us in the verse prior to Pilate’s question: “For this I have been born, and for this I have come into the world, to testify to the truth. Everyone who is of the truth hears My voice.” The Lord tells Moses and Israel: “The LORD, the LORD God, compassionate and gracious, slow to anger, and abounding in lovingkindness and truth” The Psalmist calls the Lord the “God of truth.” Truth is over and over said to be God’s, and to be something He Himself is. His works are themselves truth. His word is called truth, repeatedly. That truth is said to be “everlasting.” Further, to His people, there is another promise: ““I will ask the Father, and He will give you another Helper, that He may be with you forever; that is the Spirit of truth, whom the world cannot receive, because it does not see Him or know Him, but you know Him because He abides with you and will be in you.”
Romans speaks of those who suppress the truth; “For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men who suppress the truth in unrighteousness,” It goes on to speak of what that suppression involves; “For they exchanged the truth of God for a lie, and worshiped and served the creature rather than the Creator, who is blessed forever. Amen.” Paul tells us in 2nd Timothy that repentance leads to knowledge of the truth – that the unrepentant are always “learning”, “and never able to come to the knowledge of the truth.”
When thinking of truth, you simply cannot do so apart from considering consistency. Thus, when you consider the truth of the religious system, you should consider the consistency of it. Many objections we encounter, when boiled down, are objections on the basis of consistency. That which is internally inconsistent – inconsistent with itself – is untrue. What this must be distinguished from is inconsistency with a) your own system, as distinct from the system under consideration – and b) your own conceptions of the system under consideration, not the system itself. The former is also called an “external” inconsistency. It may certainly be true that this system is incompatible with your own – but that’s an irrelevancy, at best. You should be measuring the “consistency” of the system in terms of the “rules” from that system. Too often, a system is externally evaluated *as if* it operates by the rules the evaluator operates under. You have to be cognizant of the difference between the two. Bahnsen, for example, calls this being “epistemologically self-conscious”. Epistemology, as you may have gathered, is the study of how you know what you know. In some sense, when you confuse the two, you are comparing apples to oranges. It is more of a matter of misunderstanding than conflation, if you are interpreting the other system as if it worked within the framework of your own. When it “fails” to be consistent in those terms, it is asserted that it “fails” – but what really just happened is that it failed to be what you think it is.
To be truly inconsistent, it needs to have contradictory tenets that the system itself says should be contradictory. That is called an “internal” inconsistency.
For instance; the Qu’ran asserts that the people of the book should judge by what is therein, in Surah 5. “And let the People of the Gospel judge by what God has revealed in it. If any fail to judge by what God has revealed, they are licentious.” In Surah 10, Muhammad is told “So if you are in doubt, [O Muhammad], about that which We have revealed to you, then ask those who have been reading the Scripture before you. The truth has certainly come to you from your Lord, so never be among the doubters.” If a proof, to the Prophet, is supplied by means of asking us what *we* believe, and what we have read, it is safe to say that the Christian Scriptures are considered trustworthy. However, there is a problem – an internal problem. Modern Islamic teachers state that the Christian Scriptures were corrupted. This is an inconsistency between faith and practice. On the one hand, the Qu’ran, in many places, attests to the reliability of the Scriptures which it claims to guard. On the other, we are told that the Qu’ran failed at it’s task. That nothing remains of the former words of Allah, which the Qu’ran affirms cannot be broken, or fail. The Injeel is lost, known only to Jesus. Further, the Qu’ran itself fails to properly identify what Christians have believed, from the beginning. When the Qu’ran states: “And when Allah will say, ‘O Jesus, Son of Mary, did you say to the people, ‘Take me and my mother as deities besides Allah ?”” It identifies the Christian Trinity as Father, Son, and Mary! If the author truly knew Christianity, and what their Scripture teaches, would they not know this is incorrect? These are internal inconsistencies, and are a gauge of truth claims.
When we speak of “the impossibility of the contrary” elsewhere, and the negative argument it posits, we are speaking, in part, of this topic. Islam, therefore, is self-contradictory, and thus cannot be true. This procedure is necessary to eliminate the false. Which brings us to the next question.