For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and divinity; so that they are without excuse: Rom 1:20 Evidence of creation and Intelligent Design is everywhere and it is undeniable.

According to this verse everyone has a Notion Of God
   ➛ that He exists
   ➛ that He is Creator

It is this Notion Of God along with The Measure Of Faith that He imparts to all human beings that is a nudge from Him in His direction.   

Unbelievers raise the question:
"If there is a God why doesn't show Himself!?"

He did Gal 4:4 .   

But what they are asking for is a constant manifestation of an impressive and imposing figure to be constantly in their sight doing earth-shaking miracles.   

But that is NOT what they want.   Because such a God would control them preventing them from rebellion or swiftly punishing them for disobedience.   

So, many choose to disbelieve in a God who made a brief showing on earth to a little country in the middle east, who made some kind of sacrifice of Himself leaving others to represent Him.   

Couple such Carnal Minded unbelief with the weakness amongst believers in God and you are left with planet in a royal mess.   

Jesus came to solidify the Notion Of God by
   ➛ His Word
   ➛ His Holy Spirit

But the witness of His Word and Spirit in His own life was not the end-all.