Use the verb espouse to describe the actions of someone who lives according to specific beliefs, such as your friends who espouse environmentalism and as a result walk whenever possible instead of taking the car.

You can see the word spouse in espouse, so you may be wondering what husbands and wives have to do with it. Originally espouse did mean "to marry," but its meaning has evolved to include other long-term commitments as well, such as support for a principle or a cause. Similar to marriage, if you espouse a belief system, the idea is that you've chosen to wed yourself to it.

Definitions of espouse
  1. verb
    choose and follow; as of theories, ideas, policies, strategies or plans
    “The candidate espouses Republican ideals”
    synonyms: adopt, follow
    abide by, comply, follow
    act in accordance with someone's rules, commands, or wishes
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    adhere, stick
    be a devoted follower or supporter
    type of:
    choose, pick out, select, take
    pick out, select, or choose from a number of alternatives
  2. verb
    take up the cause, ideology, practice, method, of someone and use it as one's own
    synonyms: adopt, embrace, sweep up
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    fasten on, hook on, latch on, seize on, take up
    type of:
    consider or hold as true
  3. verb
    take in marriage
    synonyms: conjoin, get hitched with, get married, hook up with, marry, wed
    marry, splice, tie, wed
    perform a marriage ceremony
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    marry within one's own tribe or group
    marry an unsuitable partner
    marry a woman, take a wife
    take (someone) as a wife
    marry within the same ethnic, social, or family group
    marry, not for the first time
    type of:
    unify, unite
    act in concert or unite in a common purpose or belief
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