The 78-year-old Pope, referring to the growth of new sects, said there was "a kind of new explosion of religion" that if pushed too far, turned religion into "almost a consumer product."
Issuing a staunch defense of the everyday practice of religion, the Pope warned the multinational crowd of young pilgrims that constructing their religion on a "do-it-yourself" basis would ultimately prove fruitless.
"People choose what they like, and some are even able to make a profit from it. But religion constructed on a ‘do-it-yourself’ basis cannot ultimately help us."
"It may be comfortable, but at times of crisis we are left to ourselves."
Many in the huge multi-hued crowd, including Asians and Africans as well as Europeans, Latin Americans and Americans, spent the night at the site, a former coal mine on the outskirts of Cologne, after a prayer-vigil with the Pope the night before.
Nevertheless, most roused themselves in time to give Pope Benedict a noisy reception under a massed canopy of swirling flags when he arrived aboard his popemobile, waving and blessing the crowd.
Before beginning the Mass, the Pope raised a huge cheer when he greeted the crowd, saying: "I would like to have been able to go through the crowd and greet each and every one of you."
The crowd applauded the Pope at the end of his 20-minute homily, which concentrated heavily on spiritual matters. The pontiff urged the young Catholics to keep God at the center of their lives and underlined the importance of Sunday Mass and receiving Holy Communion.
Holy Communion, in which Catholics believe that bread and wine is turned into the body and blood of Christ in imitation of the Last Supper, was like "nuclear fission," he told them.
At one point, he cajoled them like a grandfather, gently urging them to "make the effort" to attend Sunday Mass.
"Dear friends, sometimes our initial impression is that having to include time for Mass on a Sunday is rather inconvenient. But if you make the effort, you will realize that this is what gives a proper focus to your free time.
"Do not be deterred from taking part in the Mass, and help others to discover it too," the Pope said.
The Mass is the climax of a successful four-day visit to his homeland, the first foreign trip of his pontificate, which begun on April 19.
Benedict has used the trip to largely bridge a gap with the young, who for the most part seem to have been won over by the new pontiff, even if he lacks the charisma of the late John Paul II, who even in his later years reveled in such opportunities to meet young Catholics.