The theologian and youth worker told the Christian Resources Exhibition in Esher, Surrey, that young people were far likelier to absorb the story of secularism than they were the story of Christianity.
With an estimated 70 per cent drop out rate among young Christians in the Western church, he said there was a challenge to bring young people deeper into their discipleship and move from passive to active faith.
This requires that the church regard youth work as more than a "tag on".
"We suddenly wonder why when they hit age 18 or their twenties they are not reintegrating into mainstream church," he said.
"It's because we made them used to an entirely different style of church that's really relevant for them, and then we try and graft them into a radically different experience of church than the one they've grown up with.
"From zero to 18, they might not sit through a normal church service or celebrate worship with the adult family and then we expect them to suddenly change to our style of church. It's a big shift."
He echoed the concerns of sociologist Tony Campolo, who warned that the church had perhaps made Christianity "too easy" for young people.
"We come along to a cell group once a week and we come to Sunday service. Is this what Christianity is or are we setting the bar too low for them and they don't want to grab it?" said Gardner.
To involve young people in the life of the church, he said it was going to take more than giving them the job of handing out the hymnbooks.
"It's the culture of the church that sends out the right messages that say we want you on board and not just in a token sense, a youth service once a month, but we want you to be right at the heart of what we are doing as a church."
In addition to greater involvement, Gardner said there was a strong desire for affirmation among young people.
In a world where 750 million people are Facebook users, the internet has moved on from being a resource for information, to providing an extended community. It's interactive, it is a "peer experience", and most of all, it eliminates the need to meet in a physical place.
"You don't need to go to church to stay connected or in touch. You have an iPhone," he explained.
"We've got increasing competition, struggling to get young people along to events because a better option might come up four seconds before that event starts.
"It is becoming increasingly difficult to create that physical community."
He continued: "As family ties have weakened we have strengthened peer affiliations. In youth ministry, we've lagged behind the changes we need to make."
Although young people are looking for affirmation, they often don't find it in churches, he said, because "we separate them out too much, we separate them into peers, and we haven't given them that rich feeding where they learn from the older people within our community".
The church was also failing, he said, to train young people for life outside the church and the inevitable confrontation with other perspectives.
The result is a "genie God" faith that sees God as a distant problem solver and the call of the Christian faith reduced to "being nice".
"What kind of spirituality are we creating?" he asked.
"Church has been impacted by consumer culture, by a mentality that says our lives now revolve around the absence of God rather than the centrality of God.
"The means and channels through which we embrace the absence of God are far more abundant than the means through which we are actually absorbing the Christian story.
"Our culture is doing a better job of discipleship than our churches are."
In the face of such challenges, Gardner called for family worship to be strengthened as parents are still the biggest predictor of the spiritual lives of teenagers.
He pointed to the continued growth of Mormons in the US and their rich culture of family worship, with some Mormon children being getting up at 5am each morning for two hours of Bible study before school.
"We are nowhere near that dedicated in terms of telling the Christian story," he said.
With "too much complacency and too much apathy" in the Western church, he said there was a need for older people in the church to model devotion.
"This is a transition we are going to have to wake up to but it's not about making parents guilty about what happens in the home. It's about seeing an evolution towards family worshipping together."