"since what may be known about God is plain to them, because God has made it plain to them. For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse."  Romans 1:19-20
I am not defending Catholicism - it has had/does have many problems where its doctrine and practice has departed from plain Bible New Testament teaching, but God's existence and Biblical Christianity are a different issue. 
http://biblereasons.com/atheism/