The decision to accept the Bible as God's infallible Word was based on human reason and feeling. You only have a uniquely moral foundation if that decision turns out to have been perfectly right. If that decision was mistaken, then the worldview that you call Christian is in fact a mixture of right and wrong, good and bad. And that decision to accept the Bible as infallible was based on flawed things, so it easily could be mistaken.
So your moral foundation is no more secure than anyone else's. One difference is that because you spent your entire capital of human reason and conscience in one big lump sum, to buy the infallibility of the Bible, you can more easily forget that all your conclusions are still fallible, the way somebody who buys an expensive vacation at an all-inclusive resort, paying for everything in advance, can then easily forget that everything still costs money, because once they've paid their way into the resort everything there is just free.
Just because your worldview says that your worldview is the only moral one doesn't mean that it really is. Here's a rival moral foundation that consists of two principles.
If you somehow accept these two principles, then you will be believing that you have a perfect moral foundation, because that's what Principle 2 says you have. This moral foundation is nevertheless absurd. Just because your system claims it is perfect doesn't mean that you really have a perfect system.1) Whenever there's a moral decision to be made, flip a coin. The mysterious workings of fate will ensure that the coin toss of a true believer will always show the morally right choice.
2) These two principles are the necessary and infallible truth and it is a sin not to believe and obey them.