Avowed comes to English rom Old French avouer ‘acknowledge’, from Latin advocare ‘summon in defense’.  It first came to Middle English as the word "avouch" (affirm or vouch), later to become the word avowed.  

The only known use of the adjective avowed is in the early 1700s. OED's only evidence for avowed is from 1720, in a translation by Richard Welton, clergyman of the nonjuring Church of England. 

Examples of avowed used in a sentence:  
"The recently converted man avowed his change of faith"
"My grandfather avowed that he was no longer in perfect health"
"They're avowed enemies of the current Cuban state."
"We are endangered from the outside by our avowed enemies."
"The indulgence was a consequence of the context, of the avowed project."
"Likewise Montesquieu in his Essay on Taste owes him more than is avowed."
"The shopper confirmed that he is an avowed believer in organic farming"
"The preacher is an avowed follower of the ideology" 
"The leaders came to power with the avowed aim of promoting religious toleration"

One of the wonderful things about English is that an individual word is not necessarily specifically tied to a particular grammatical function, unlike in many other languages. Lots of English words are used to fulfill more than one grammatical function.

@Myr posted avowed as an adjective and @Bill W has as usual collected excellent background for the word avowed as an adjective along with examples of its use in sentences. In seven of the sentences avowed is clearly used as an adjective. In the first two, and @Myr's example avowed is used as a verb (past tense of avow*) describing an action rather than, as an adjective, modifying a noun.

Additionally, if you google (a word which is both a verb and a noun) avowed you will find that Avowed is also a video game and hence now also a noun.

So here we have avowed not only as an adjective but also as a verb and a noun. Isn't English wonderful!

(* avow has become both the noun and verb vow as used in weddings.)

Just the facts:

AVOWED Definition & Meaning - Merriam-Webster