John 14:6. I am the way, the truth and the life
John 18:38. Pilate said to him, "What is truth?"
Doubts can easily come to mind if you rely on your natural senses, the ones that you use to make sense of this world. But beware! The bible tells us that Satan is the ruler of this world. Instead we access the kingdom of God and His truth with our spiritual senses:
And yet there is no direct comparison with our five natural senses, although sometimes it happens that the spiritual world comes so close that people have seen with their natural eyes a vision of Christ, or heard heavenly music, or felt the touch of the hand of God on their bodies, or smelt heavenly perfume.
No, normally, we communicate with God through Christ in our spirits: your spiritual ears that hear the word of God spoken into your spirit; your spiritual eyes that see something about another life.
And that requires faith to believe in the reality of the heavenly kingdom, that it is more real than the natural, for Christ has overcome the world. (John 16:33)
Once having seen and known the truth with our spiritual sight, we should never be able to 'unsee' it, we must not let this truth be confused or dimmed by relying on our natural senses.