It is no surprise that the vast majority of the world believes differently than you do.
The vast majority of the world believes all sorts of things. It isn't like the world outside of Christianity is unified in its beliefs. Far, far from it.
Why is this? Why are so many people being deceived by other religions if they clearly aren't true? Are billions of people just morons who are suppressing the truth? If the one true God is so clearly and plainly visible, why are people following other ones? Some people are dedicating their entire lives to other religions and some even blowing themselves up in the name of religion.
The Bible is pretty clear about its answer to your questions:
Ephesians 2:1-3 (NKJV)
1 And you He made alive, who were dead in trespasses and sins,
2 in which you once walked according to the course of this world, according to the prince of the power of the air, the spirit who now works in the sons of disobedience,
3 among whom also we all once conducted ourselves in the lusts of our flesh, fulfilling the desires of the flesh and of the mind, and were by nature children of wrath, just as the others.
2 Corinthians 4:3-4 (NKJV)
3 But even if our gospel is veiled, it is veiled to those who are perishing,
4whose minds the god of this age has blinded, who do not believe, lest the light of the gospel of the glory of Christ, who is the image of God, should shine on them.
As the apostle Paul explains above, people are induced to believe the wrong things because the "god of this world," the "prince of the power of the air," the devil, has blinded them to the truth and has deceived them into living according to his lies. But, this isn't the only reason why so many people believe the wrong thing. Paul also points out that those who have not had a second
spiritual birth are "dead in trespasses and sins." They are without the spiritual capacity, given only by God's Holy Spirit, to understand God's truth.
1 Corinthians 2:12-14 (NKJV)
12 Now we have received, not the spirit of the world, but the Spirit who is from God, that we might know the things that have been freely given to us by God.
13 These things we also speak, not in words which man's wisdom teaches but which the Holy Spirit teaches, comparing spiritual things with spiritual.
14 But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned.
Finally, humans have a natural inclination away from God. Human pride and selfishness provoke us to reject God's claim upon us. We want to live our own way, not God's. We don't want to be told what we are doing is wrong. We don't want to have to answer to our Maker. We want to gratify ourselves and God gets in the way. Jesus explains:
John 3:19-20 (NKJV)
19 And this is the condemnation, that the light has come into the world, and men loved darkness rather than light, because their deeds were evil.
20 For everyone practicing evil hates the light and does not come to the light, lest his deeds should be exposed.
"Those who practice evil" describes every person on the planet. As Paul points out,
Romans 3:23 (NKJV)
23 for all have sinned and fall short of the glory of God,
Thus, as beings who practice evil, all humans resist God's reproof of their evil deeds. And in an attempt to do so, humans erect counterfeit or surrogate gods and systems of belief that better suit them.
Paul further notes that not only do we naturally avoid God's correction but we just don't have it in us as carnal people "dead in trespasses and sins" to live in a godly way. A carnal person, one who is living according to the dictates of their fleshly needs and desires rather than under the direction of God's Spirit, is totally incapable of living in a manner pleasing to God.
Romans 8:7-8 (NKJV)
7 Because the carnal mind is enmity against God; for it is not subject to the law of God, nor indeed can be.
8 So then, those who are in the flesh cannot please God.
With all of these factors in play opposing the Gospel, it really is a miracle when a person comes to saving faith in Christ. It has to be an act of God that brings anyone to salvation. And this is why, I think, Jesus said more people would be lost than saved.
Matthew 7:14 (NKJV)
14 Because narrow is the gate and difficult is the way which leads to life, and there are few who find it.
If a bunch of other man-made religions have gained the same amount of popularity and followers as Christianity, then I think this says something about what man is capable of. Man is capable of deception at an enormous scale. Man can deceive billions of people into thinking any god exists. Therefore, why should I let what man says be the foundation for my belief in God?
As I pointed out above, it isn't just that man deceives himself. Humans have a great deal of help from the demonic in their self-deception. You must in the end be convinced for yourself that the Bible is the divine text it claims to be. There is good reason to believe it is, but it will not do to believe merely because someone else does. You must make the effort to study the Bible for yourself and come to your own conclusions. And you must have a meeting with God. If your desire is truly to know God, then He will move heaven and earth to give you that knowledge. But He is not deceived by the things we use to deceive others - and ourselves. He knows what you truly desire and He will act accordingly.
Selah.
