The existence of a prime mover- nothing can move itself; there must be a first mover. The first mover is called God.
Thomas AquinasSince faith rests upon infallible truth, and since the contrary of a truth can never be demonstrated, it is clear that the arguments brought against faith cannot be demonstrations, but are difficulties that can be answered.
Thomas AquinasCommemoration of Gilbert of Sempringham, Founder of the Gilbertine Order, 1189 Some there are who presume so far on their wits that they think themselves capable of measuring the whole nature of things by their intellect, in that they esteem all things true which they see, and false which they see not. Accordingly, in order that man's mind might be freed from this presumption, and seek the truth humbly, it was necessary that certain things far surpassing his intellect should be proposed to man by God.
Thomas Aquinas