How is Christianity true when true ancient Satanism and the ancient satanic Scriptures pre-date Christianity and Jesus himself?
I am afraid that your argument is a classic example of the logical fallacy of appeal to age. This logical fallacy is that something is right or better simply because it is older. If that were true, then nothing believed to be true in science would be true. For example, belief in geocentrism is much older than belief in heliocentrism. Does this make it true? No. The ether theory preceded the theory of relativity. Does this make it true? Again, no.
Let us assume that the “ancient satanic scriptures” pre-date Christianity. What does this prove? Nothing. If Satanism is truth and if it is right, this conclusion would be based on the evidence, not on whether it is old or not.
Besides, you presented no evidence to support your contention. What “ancient satanic scriptures” pre-date Christianity? What scriptures are you talking about? Can you quote from these supposed scriptures and can you present physical evidence that such scriptures existed before Christ? This is an empty claim as far as I can see.
But in the end, whether Jesus is who he said he is or whether worship of Satan is a good moral thing to do is to be decided on the merits of the claims of Jesus versus the merits of the claims of Satan-worshippers, not on which belief existed first. So, if you want to somehow claim that Satanism is true and Christianity is false, you need to present an actual argument rather than use a rather obvious logical fallacy as your only argument. I just posted a lesson titled “Evidence for Jesus” It is available at https://evidenceforchristianity.org/lessons-from-india-bangaladesh-and-sri-lanka/ You may want to check out this audio lesson to find evidence that Jesus is who he said he is.
I feel I may have come across a bit too strong here, and I would love to hear an actual argument for your belief being true if you care to present one.