Christians, what is your VERY BEST arguments for the existence of God?

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..which equates to the same thing in my opinion... so can you explain how you can dismiss those two arguments so I can see?

It IS NOT the same thing. Disproving two arguments for the existence of God does not mean i have disproved God entirely.

The 'first cause' argument is easy two disprove for two simple reasons, though there are many arguments that take it out:

1) It is a blatantly obvious case of special pleading, ie 'everything that has a beginning has a cause, except God'

2) Not everything that has a beginning has a cause, for example a Carbon-14 atom can spontaneously decay into Nitrogen 14 emitting ionizing particles without anything at all acting upon it, no force, no energy, nothing. It is a truly random process that is entirely uncaused. Therefore claiming that everything that has a beginning has a cause is false. Considering that, Occams razor can be applied to show that the Creator/Prime Mover requires one more assumption than the singularity while not having any basis in necessity and is therefore not the preferred explanation logically.

The argument from fine tuning is destroyed by the context and scale of the universe. The argument that the universe was made for life is rendered ridiculous by the fact that for one life supporting planet in a tiny corner of the galaxy is accompanied by incomprehensibly vast regions of empty space, galaxies with hundreds of billions of stars and a universe with hundreds of billions of galaxies. In this scale life does appear as an extremely low probability occurrence that can take place only in very rare circumstances.

the example you give of a seemingly causles process wont convince me, carbon-14 has a half life and in the fullness of time it will decompose. just because you havent identified a cause for it doesnt imply there isnt a cause. secondly the only thing thats blatanly obvious is the caricature you painted of the cosmological argument. the argument says whatever begins to exist has a cause since GOD has no beginning it doesnt apply to him. the universe was made to support life, not to create life from slimey conditions. hence the rarity of life does nothing to challenge ID.