Bias exactly the problem. Everyone has their own reality. To prove something to someone or win a debate, you must first get them to doubt their own reality. Trying to establish a new reality on top of the existing won’t work since two truths can’t exist together. First you must cause doubt in their current reality. Once the current reality is broken a new one can be established. Their faith in the new reality can now begin to grow are long as it’s not taken away by the old as in the parable of the sower. The current crop has to be removed and the ground tilled before the new crop can be planted.
And Mere Christianity is definitely a book that I would recommend for all that are doubting their current reality.
There's no such animal in this universe as multiple truths, since all truth is God's Truth.