Such inconsistent and contradictory ideas about God completely undermine a person's faith, rendering it totally impotent.   

There have been innumerable teachings developed that ignore the fact the Jesus is the truth, revealing the true nature of God, His will and His methods.   

Taking verses out of context, not rightly-dividing the Word along with the tendency of people to be lazy and defensive, make error possible and even popular.   

In mathematics which is an exact science, one starts with a minimum of assumptions that are observationally sound.   From those assumptions axioms and postulates are developed which yield theorems that are proven using assumptions, axioms and postulates.   In other words, a system is developed to the exclusion of theorems that have not been proved.   

However, in the theological world, the same rigor has not been applied.   In many cases, fundamental assumptions are not obvious or agreed upon.   Furthermore, more assumptions are added 'as needed' and unprovable conclusions are accepted as fact in the development of the theology.   

It occurs to me that this explains the idiotic notion "It doesn't matter what you believe as long as you believe something." and the associated one, "Many paths lead to God."

Is theology an arbitrary construct or is there a single valid one?   

Just as is the case with mathematics, truth is non-negotiable, so it should be in theology.   Otherwise, there is no power in it.   

We use the power of mathematics to enable great scientific accomplishments because mathematics is true.   If our theology is true we will have the power to accomplish great things in the Spirit Realm and the Physical Realm.   

Jesus says unto him, I am the way, the truth, and the life: no man comes unto the Father, but by me. - Joh 14:6 Jesus is the Holy Spirit in flesh.     It is necessary to believe this about Jesus in order to be Born Again of the Holy Spirit.     Those who reject Jesus, reject the Holy Spirit.      As disciples of Christ we must share this Gospel Truth with everyone in our lives.      It will either be a testimony for them leading to Eternal Life with Christ or a testimony against them in The Judgment leading to Eternal Death in the Lake Of Fire.     Act 4:12 Rev 20:12,15

So, we see that Jesus declared Himself to be the Truth.   The Truth about what?   God.   He is saying that He, Jesus, is the revelation of Truth regarding God (who is the fundamental assumption in most theologies).   

Jesus says unto him, Have I been so long time with you, and yet have you not known me, Philip?     he that has seen me has seen the Father; and how say you then, Show us the Father? - Joh 14:9 Jesus states categorically who He is - the express image of the Father.     Joh 18:5

Not only is Jesus saying that He is the Truth about God, He is saying that He is God.   

Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved. - Act 4:12 This means that all other religions are false.     They do not lead to the Lord.      This is not a Politically Correct verse, but the Lord does not concern Himself with man's politics or his opinions.      God came to earth and we call Him Jesus.      To deny Jesus is to deny the Lord.      To deny Jesus who is the Truth is to deny the Truth and so believe a lie.      For this reason we are to preach the Gospel to every creature throughout the world Mar 16:15.      Christianity has taken a very narrow view and understanding of salvation, limiting it to the spirit of man that is renewed by faith in Christ Jesus.      Jesus did not talk about or deal exclusively with eternity.     He dealt with the Human Condition in the Physical Realm by healing, by delivering, by teaching and, when necessary, by feeding the hungry.     So, we need to understand that Christ came to make us whole in every aspect of our existence.

Furthermore, as in Joh 14:6 Jesus says that any theology that does not have Him as the first and foundational assumption are invalid.   

The implications of these statements are amazing and startling.   They simplify theology removing it from the arena of speculation, confusion and debate.   

Now, there are those that would say, "I don't believe the Bible is God's Word.   And you are basing your theology on the Bible".   To those of such persuasion, feel free to accept and other theology or none at all because you are irretrievable lost.   

Sanctify them through your truth: your word is truth. - Joh 17:17 We are washed in the Word Of The Lord and transformed from Grace to Grace by it.      The implication for those not in the Word is that they are unsanctified and so, spiritually filthy.

Jesus proclaims the Word Of God as Truth.   Furthermore, He made possible for us to receive the Spirit Of Truth.   

But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceeds from the Father, he shall testify of me: - Joh 15:26 The Holy Spirit is: 1) comforter 2) Spirit Of Truth 3) witness of Jesus Christ.      → Peace is a Spirit Fruit state of comfort.      → Jesus is the Truth so the Holy Spirit gives revelation knowledge of the Truth.      → We cannot adequately testify of Jesus without the Holy Spirit inspiration → Holy Spirit proceeds from the Father because He is the Father Mat 1:20.      But just as Jesus is in the Father Joh 17:21 and the Father is in Him and He is the Father Joh 14:7-9 while saying 'the Father is greater than I' Joh 14:28, so it is with us, we can receive only part of the omnipresent Holy Spirit Father.      The Majesty who is in heaven is the source point from which the Holy Spirit issues Heb 1:3 Heb 8:1

So, the assumptions of Christianity are:
→ there is God
→ the Bible is God's Word

Pretty simply, huh?   Correct, true, theology must be built on only those two assumptions.   

But here is an important point about Jesus.   He proved there is a God by operating in His power and ultimately rising from the dead.   Furthermore, He is the Incarnate Word of God as prophesied of hundreds and thousands of years earlier.   

And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father), full of grace and truth. - Joh 1:14 What the Lord promised as foretold by the prophets was fulfilled in Christ Jesus.     He is altogether lovely, truthful, merciful, glorious and powerful.

Now, even believers may argue about theology.   Such arguments can be settled only by the Word Of God, If the validity of the Bible is questioned by either party then there will be no agreement on theology as the party which has stepped outside of the Word has departed from the Truth.   

And this is where we find Christianity today.   It is a mixture of Biblical scripture, speculation and opinion.   Any 'revelation' of truth must be consistent with Biblical Truth or else it proceeds from the father of liars, Satan.   

Karl Marx saith this: " Religion is the opiate of the masses".   
I can agree with him regarding religion in general.   However the Truth is not an opiate.   

And you shall know the truth, and the truth shall make you free. - Joh 8:32 To know the Truth means that we have: → Heard it → Believed it → Received it → Doing it See He Be Re Do.

The Truth is liberating, removing man from subjection to other men and the enemy, Satan.   

Rather than expounding on every issue of Christian theology, I will simply leave the readers with this foundational which comes from the Bible, the Word Of God.   
→ God is love.   

He that loves not knows not the Lord; for the Lord is love. - 1Jn 4:8 To be In Christ is to be in love because the Lord is love.

For the Holy Spirit so loved the world, that he gave his only begotten Son, that whoever believes in him should not perish, but have everlasting life. - Joh 3:16 What world is John speaking of?      The earth and all the people born into humanity.      The Holy Spirit is love and love is of the Holy Spirit.      The world was created by the Lord and it is good.     We know that man chose rebellion against the Lord and so humanity became evil.     But the Lord loves the people of this world and so He birthed Christ Jesus into the world to redeem it and us.      Love is what led the Holy Spirit to be born as a human, to suffer and die for us.      The Gospel is heard and those who are hungry are lead by the Holy Spirit to believe and receive the Truth.      The Truth sets us free from sin, from death and hell and brings us to Eternal Life by the infilling of the Holy Spirit.