Many, if not most people, think that all religions are the same, including Christianity. In fact, I would agree that there is a common core to all religions including Christianity if you are referring to the liberal Christianity that characterizes most main-line denominations today. This fact, I believe, makes the unique message of the Bible all the more a great evidence of its divine origin. Now, all religions have unique ideas making uniqueness in and of itself a poor evidence, but it is the nature of the unique message of the Bible that points to it as divine revelation from the God Who really exists.
The common core of religions is due to the common source of the religions, the human mind (one could argue that it is the human mind as influenced by Satan). Although all religions have peculiar ideas, there is a commonality in their basic concept. Not all human religions even believe in deities (forms of Buddhism, for instance) nor do all think of some kind of a heaven or hell; however, all human-originated religions, including liberal Christianity, see mankind as being on a journey to an afterlife in which man saves himself through a process of becoming what the religion says they can become and/or going to where the religion says they can go. This core is the natural conclusion of human reasoning which is essentially selfish seeking to have something in which we can boast. In this core, all religions betray their human origins.
The message of the Bible, however, is very different. God’s thoughts and ways are not the thoughts and ways of man (Isaiah 55:8, 9) nor can man by his natural reasoning understand the message of God (I Corinthians 2:14). Therefore, we would expect God’s revelation to be different in essence from all the ideas of man, and that is exactly what we find in the Bible. There are a number of peculiar ideas taught in the Bible. First, it reveals God in a way no other foundational religious writing does. Man-made religions have either no god, many gods, or a single-person god. Only the Bible teaches a Trinity, one God made of three Persons. This concept has been debated and explained for two thousand years. The Trinity is denied even by the so-called Christian cults and liberal Christianity. However, it is clearly taught in Scripture (Genesis 1:26; 19:24; Matthew 3:16, 17; 28:19; John 14:16-18, 24-26; II Corinthians 13:14 etc.). It is very difficult to understand because, I believe, it is not an invention of man but part of that which is “higher than” our thoughts. We can understand the inventions of our own minds because we invented them; but we cannot understand the revelation of God by our own natural reasoning because such are revelations of God Who is infinitely greater than us. Another example of the unique teachings of the Bible is its revelation of the person of Jesus Christ. Polytheistic religions refer to gods taking on the appearance of humans and even producing children with humans who then would be half-god and half-human, but not gods actually becoming human. Not only so, but these deities appear as humans and interact with them out of the same selfish motives so commonly found in humans; but the Second Person of the Trinity became man to save man (John 3:17; 10:47), to identify with us (Philippians 2:6-8; Hebrews 4:15; 5:1-9), to be our Representative (Romans 5:12-21; I Corinthians 15:21, 22), to bear our sin (Isaiah 53:4-6; II Corinthians 5:21; I Peter 2:24), and to take the just wrath of God against our sin in our place (II Corinthians 5:21; Galatians 3:13) and become our great High Priest (Hebrews 5-10) all out of His love for the humanity that rebelled against Him (John 3:16; Romans 5:8; Ephesians 2:4, 5). The fact that He did so without giving up any of His deity (John 1:1-3, 14, 18; 20:28; Romans 9:5; Colossians 1:13-19:2:9; I Timothy 3:13; Titus 2:13; Hebrews 1:1-3, 8) is another example of a concept that has been debated and explained for two thousand years, again, I believe, because this concept is also beyond human thought. This hypostatic union of God and man in Jesus Christ is unparalleled in other systems of thought. The Bible also depicts man in a unique way. Man is presented as being made in the image of God (Genesis 1:26, 27), but fell into sin by direct disobedience (Genesis 3; Romans 5:12, 17, 19), and is therefore sinful and so worthy of eternal judgment because of his own actions (Isaiah 53:6; 59:1, 2; John 3:36; Romans 1:18; 3:10-19; Galatians 3:10; James 2:10), and unable to contribute in any way to his own salvation (Romans 3:20; 4:4, 5, 16; Galatians 2:16, 3:21,22; Ephesians 2:8, 9; Titus 3:5, 6). This portrayal carries over even in its presentations of great men such as the Patriarchs, Moses, and David revealing their sins and defeats. Ancient works seldom described their heroes so and certainly rulers never depicted themselves in such a way. But above all other unique concepts is the biblical doctrine of salvation. The Bible clearly teaches that we can do nothing to save ourselves (Ephesians 2:8, 9). Salvation is a gracious work of God granted on the basis, but not the merit, of repentance and faith in the Lord Jesus Christ and His finished work and Resurrection for us (Isaiah 55:6, 7; John 3:16; Acts 3:19; 16:31; 17:30; Romans 10:9-13). In all other religions, including liberal Christianity, salvation (or its equivalent) is achieved by man’s own efforts. In some Eastern religions, salvation requires even many life-times of effort. This is the natural reasoning of natural man who always wants a reason to boast (Ephesians 2:9). Only divine revelation would create a means of salvation that is fully and only by the grace of God Who loves us and gave Himself for us so that all glory goes to Him alone.
I want to reiterate that the fact that Christianity has unique aspects is not my argument. All religions have unique aspects. My point is that the teachings of the Bible are unique in a way that reveals it is a revelation of God. It is unique from the core story of all other religions in a way that is contrary to the nature and thinking of humans. The core teachings of other religions and philosophy demonstrate the basic pride the Bible shows is part of fallen human nature; the teachings of the Bible are contrary to human pride and would never be devised by fallen man. The question should then be asked as to what is behind this phenomenon. Why are there no other religions which have their core teachings like the Bible? What was the origin of these teachings? The Bible itself points out that natural man could never devise such a core body of doctrines (I Corinthians 2:6-9). Instead, they demonstrate the revelation of the God Who is there (I Corinthians 2:10) and can only be truly understood as we accept the salvation revealed by God and so receive the Holy Spirit and are taught by Him (I Corinthians 2:10-16). The very teachings of the Bible, therefore, demonstrate that it is indeed the revelation of God.