Faith is the confidence that what we hope for will actually happen; it gives us assurance about things we cannot see.
It is impossible to please God without faith. Anyone who wants to come to Him must believe that God exists and that He rewards those that sincerely seek Him. (Hebrews 11:1, 6 NLT)
When people ask me what we have to believe I have often said the Gospel requires very little faith in reality, at least to start with. All we need to believe is that God exists and that He is good. Everything else can be logically built upon those two foundations. If God exists and is good then it follows that He would do all that the bible says He has done.
No one can prove God exists but then neither can any of us prove that anything ultimately exists since we cannot know the ultimate materials on which matter is built. The more we split the atom, the more there is to split. So, rather than being unreasonable, we logically believe in some basic building blocks (atoms, nucleus, protons, neutrons and electrons) and our engineered inventions are built on those beliefs. They work because the things we believe about materials are true.
In the same way, the bible tells us to be reasonable. Believe in some basic things: God is and He is good. After that, everything God has caused to be written in His book is reasonable and makes sense. However if God does not exist or, if He does but He is in fact evil, then the bible is either rubbish or a perverse set of stories of an evil being with malicious intent towards us all.
God says: “I AM”. He does exist.
The bible goes onto say that God gave His only Son that whoever believes in Him should not perish but have eternal life. He is not willing that anyone should perish. In other words He is good.