The Bible begins with God creating the heavens and the earth – the entire universe.  God existed before the creation of the universe, but beyond that fact, the Bible does not address anything before God’s act of creation.  It does not offer any formal proof for God’s existence.  What scripture does give is a reason to believe in God’s existence.  The fact that we exist and this world exists is given as evidence that God exists.
This type of evidence for God, the existence of human beings and the world, is organized in several ways.  Two of these are the cosmological argument and the teleological argument.
The cosmological argument can be summarized as one of cause and effect.  Anything that happens is caused by something else.  You can’t have an infinite line of cause and effect because, practically, that would mean nothing would actually ever get started.  But even the mainstream scientific theories that explain observations of the universe suggest there must have been a finite beginning to everything.
The Bible says that beginning was not a random accident, but the purposeful action of a rational God.  Outside of religious thought, the world wants to accept a beginning by random chance while simultaneously seeking meaning and purpose to life.  Scripture affirms, first of all things, that the universe has a meaningful and purposeful beginning.  Not only a purposeful beginning, but a meaningful destiny.  The world did not begin by accident and it will not end by accident, either.
The doctrine of God as Creator is behind the basic theme of all scripture: This world is here by God’s deliberate intention, He sent Jesus into this world to die for our sins according to His own purpose and foreknowledge, and this world will come to a conclusion that God will bring about.
Cosmological arguments do have some weaknesses, but the Bible gives this argument as a reason to believe.  Perhaps it cannot be given with complete philosophical precision, but neither can it be disproved.  The existence of alternative ideas is not the same thing as disproving an idea.
The teleological argument can be summarized as one of purpose and design in the world.  Teleological arguments are based on recognizable order, purpose, and pattern in the midst of a decaying world.  The universe is not random.  It has regular and predictable behavior, and it is finely tuned to support the life found on Earth.
Teleological arguments have objections that point out elements in the world that do seem random and the problem of evil.  However, scripture provides explanations for what we observe without losing purpose and meaning in life.  What was originally created was called very good.  Then sin came to the world through Adam, and both human beings and creation itself were forced to suffer under a curse.  What was created good now has elements of decay and chaos and destruction.
These rational proofs do not prove God’s existence, but they are evidences that support what the Bible teaches.  What is observed in the universe and in human behavior is explained in scripture.  Is it proof?  Maybe not, but it is not foolish to believe and neither is it irrational.  There are reasons to believe that are found in the nature of creation.