It's not surprising that people are a bit skeptical about the authenticity of Justin Bieber's engagement to Hailey Baldwin. People might ask if the 25-year-old pop star and his 21-year-old model girlfriend are too young, or too emotionally stunted by fame, to be talking marriage, life's ultimate commitment.

Then there's the way Bieber and Baldwin keep ending up in paparazzi photos. That includes earlier this week when TMZ photographed them weeping together in a park in New York City, where they were riding bikes.

Sure, the paparazzi pursue these young, pretty lovebirds. But sometimes it seems as if the lovebirds are happy to be caught, as if some grander strategy for Bieber's branding efforts is at play. One cynical writer asked if the two's frequent public outings will be featured in Bieber's next music video.

But perhaps cynics should reserve judgement and note the book Bieber was carrying when photographed Wednesday.

The book is "The Meaning of Marriage: Facing the Complexities of Commitment with the Wisdom of God." Bieber flashed it to TMZ as a way of showing that he is, in fact, taking his impending marriage very vows seriously.

The tome comes from New York Times best-selling author Timothy Keller, a pastor and theologian who lectures widely on how to apply the Bible and Christian principles to building loving, committed relationships. One of those principles, Keller argues, is that people should save sex for marriage.

Bieber, who has spoken about his Christian faith and love for God, displayed the book to TMZ to explain why he was so overwhelmed by emotion during his bike ride with Baldwin the day before. He said Keller's wisdom has taught him important lessons on building a strong relationship.

"You got good days and you got bad days," he told TMZ. "It's not real if it doesn't have any bad days."

Among the book's other teachings is that the Bible confines sex to marriage — which raises the question of whether Bieber and Baldwin are waiting for marriage to become intimate.

Actually, if they have taken a chastity vow, per Keller's teachings, they wouldn't be the first celebrity couple to do so. Even in the usual pleasure-seeking culture of Hollywood and entertainment, prominent couples like Russell Wilson and Ciara, or Miranda Kerr and Snapchat billionaire Evan Spiegel, were public about their desire to wait for marriage to consummate their relationships.

In lectures available on YouTube and other sites, Keller explains why it's harmful to people's emotional and spiritual health to have premarital sex or to co-habitate.

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"Sex inside a committed marriage is magic. It's like blowing on the coals of this incredible, beautiful and powerful flame," said Keller, who founded the Redeemer Presbyterian Church in Manhattan in 2008, specifically to appeal to young, urban followers.

By contrast, Keller explained how sex outside of marriage is just a self-centered way of holding yourself back and, of only getting fulfillment, but not giving. He added that indulging in porn or sex outside marriage is "like an addiction." The more you do it, he said, the "less payoff there is."

"I can just personally say after years and years and years of marriage, of doing this with my wife, with one woman, it becomes sweeter and more pleasurable and more powerful and more transforming as the years go, even if actually neither of you look as good as you did before," said the 67-year-old Keller.

But in another lecture, Keller described marriage as something holy, as an act that God invented as a way of saying to somebody else: "I belong totally and exclusively to you."

Keller added that sexual relations are part of the covenant of marriage. Every time a married couple have sex, they are essentially renewing that covenant and "complete one-ness that happens when you get married," he added.

As has been previously reported, Bieber and Baldwin share a devotion to God and were brought together by their membership in the celebrity-friendly evangelical Hillsong Church in Los Angeles.

"Justin has a really deep faith, and he truly loves God," a source told People in 2017. Meanwhile, Baldwin's father, Stephen Baldwin, is a former actor and born-again Christian who now runs a radio ministry.

When announcing their engagement, Baldwin tweeted her gratitude to God "for giving me such an incredible person to share my life with!"

In his Instagram announcement, Bieber wrote as if his love for Baldwin and their engagement was the work of divine intervention. Bieber also cited a biblical passage, Proverbs 18:22, which is interpreted as explaining how God created marriage and believes that marriage is important for the individual and for society.

Bieber wrote: "He who finds a wife finds a good thing and obtains FAVOR from the Lord!" Bieber ended his post: "This is the year of favor!!!!"