"Wherefore should the heathen say, Where is now their God?" (Psalm 115:2)
Wherefore indeed? This question seems to be both a declaration of unbelief on the part of the unbeliever and also a plea for true belief on the part of the Christian. If we're doing our job, shouldn't God be evident in the world? Even though people choose not to believe in God, couldn't it be possible for them to know? Hmm...
With reference to prophecy, Paul says this: "But if all prophesy, and there come in one that believeth not, or one unlearned, he is convinced of all, he is judged of all:" Listen: "And thus are the secrets of his heart made manifest; and so falling down on his face he will worship God, and report that God is in you of a truth." (1 Corinthians 14:25, emphasis mine) Notice that. Paul says that if we prophesy, even unbelievers will acknowledge that God is real and active in His church. This is why it's important to respect and welcome the gifts of the spirit. Yet even prophets get the blues: Joel asks, "wherefore should they say among the people, Where is their God?" (Joel 2:18)
The same question is repeated throughout the Bible. Asaph asks in Psalm 79 (verse 10). "Wherefore should the heathen say, where is their God?" Then again, what's their motive? "When Herod saw Jesus, he was exceeding glad: for he was desirous to see Him of a long season, because he had heard many things of Him; and he hoped to have seen some miracle done by Him." (Luke 23:8) After he cross-examined Him—to which Jesus said nothing—he sent Him to Pontius Pilate. Herod's heart was not right with God, so Jesus did not reveal anything to him i.e. show him cool stuff.
Unfortunately, this seems to be the standard among non-believers who demand to see God—the God that we worship.
In John 13:35, Jesus says "By this shall all men know that ye are my disciples, if ye have love one to another." The name "Christian" carries the connotation of perfect in many circles of society. It's too bad that the lines haven't been drawn between belief and unbelief. And by the same token, people have this notion that every Christian is supposed to be a perfect specimen of the moral law. But it's not about doing all the right things. Sure James (2:18) says "I will shew thee my faith by my works", and this is good. But seeing God, as Jesus says, requires belief: "Said I not unto thee, that, if thou wouldest believe, thou shouldest see the glory of God?" (John 11:40)
"Where is now their God?" The next verse of Psalm 115 (verse 3) answers it beautifully. "But our God is in the heavens: He hath done whatsoever He hath pleased."
Gotta believe it to see it.