It should be obvious that the Gospel of Jesus Christ is to be found in the Bible, the Word of God.
The reasonableness of this conclusion is to be found in at least four ways. First, any religious group can claim it speaks for God. In fact, all of them make this claim. The very plurality means that someone is lying for the simple reason that God would have to be as confused as men to be behind all of the religions that men have dreamed up. He is not confused. So, what method has He given us to sort out the confusion for ourselves? What method do we have of checking the various claims to see who is lying? If we simply have to submit to claims without having a way to check their accuracy, we are left wide open to any deceivers who would have us follow them.
Second, most religious groups who claim to speak for the Christian God appeal to the Bible for support for their claims. For example, even the recent Mormon ads on TV appeal to part of the text of John 10 in order to establish their claim that the Book of Mormon is "another testament of the Savior". This appeal to the Bible proves that the Bible has authority over their claims. That means the Bible is the final authority.
Third, even those groups who claim that their interpretation of the Bible is infallible and authoritative must first accept the Bible as authoritative over them. Authoritative revelation precedes authoritative interpretation. Then the question is a matter of whether the Bible teaches that God has subjected us to infallible interpreters. Obviously infallible interpreters are going to try to find biblical texts that teach that God has set infallible interpreters over us to tell us what God really meant by what is clearly written in plain English in the Bible. But you and I still have to make the final decision as to whether we believe the Bible actually says what they claim it says. So the Bible still stands as authoritative and we are still responsible for the choice we make as to whose interpretation we are going to follow.
Fourth, the judgment of God is individual. This means that when I stand before Him, He is going to ask me why I believed whatever it is I believed. If my answer is, "My [church, pastor, priest, rabbi, guru, or whatever] told me that You said thus and so", what will I say when He says in response, "I made sure you had My Word in your language; I made sure that you had a sufficient education to learn to read your language; I said in My Word in your language which you could read that if you would seek to know Me through My Word I would enable you to understand what I said; so how is it that you refused to let Me speak to you by My Word and put your [church, pastor, priest, rabbi, guru, or whatever] in My place?" What will I say? What will you say when He asks you the same question? You see, I am not the only one who will face God in the judgment. You will too. He isn't going to just ask me what I believed and why. He is going to ask you too.