Synonyms for Affirm:

n.

recognize, recommend, accredit, O.K., look kindly on something, condone, give something/someone the green light. get behind, align, ally, promote, agree with, argue, speak up for, sympathize. prove, adopt, law, show, make/prove your/a point, pass, vouch for, point to. appear, arraign, allow, bring, bring before, appeal, call. affirm (noun)
corroborate, substantiate, swear, aver, support, swan, confirm, avow, assert, sustain, verify.
assert (noun)
study atassert, repeat.

v.

affirm (verb)
declare, assure, ratify, testify, attest, maintain, submit, claim, contend, warrant, acknowledge, admit, corroborate, proclaim, avouch, set down, pronounce, validate, certify, state, profess, express, support, assert, aver, adjure, propound, sustain, verify.
agree (verb)
concur, accept, assent, approve, acquiesce, cooperate, conform, capitulate, contract, endorse, welcome, consent, stipulate, promise, accede, sanction, pledge, empathize, bless, agree.
approve (verb)
authorize, permit, authenticate, underwrite, confirm, approbate.
authorize (verb)
empower, commission, entitle, denominate, notarize, designate, enfranchise, deputize, countersign, license, ordain, enable.
believe (verb)
hold, understand, judge, conclude, presume, assume, put faith in, credit, trust, gather, take for granted, surmise, give faith, suppose, deem, believe.
cognition (verb)
substantiate, sustain, support, corroborate, confirm.
declare the truth of something (verb)
profess, maintain, confirm, certify, testify, swear, vouch, ratify, aver, avouch, pronounce, declare, state, avow, guarantee, repeat, attest, assert.
guarantee (verb)
guarantee, insure, bond, safeguard, vouch, ensure, secure.

Other synonyms:

arraign. appear, adopt. recognize, call. allow. pass. approve
get behind, empower.
believe
understand, take for granted.
guarantee
ensure.
justify
prove.
promise
assure.

Usage examples for affirm

  1. The Countess of Cressett, having her cousin, the beautiful Mrs. Kirby- Levellier, in her house, has denied Lord Brailstone admission at her door, we can affirm – The Complete Project Gutenberg Works of George Meredith by George Meredith
  2. It is only to affirm that here is something not big enough for the whole either of truth or experience. – Modern Religious Cults and Movements by Gaius Glenn Atkins
  3. Nearly as much so as the potato bug, said Vaura in an undertone to Trevalyon; aloud, she said gaily: No, I rebel, and most solemnly affirm that, as you tell me Mrs. Haughton says I cultivate no pet sins, and as she is your oracle, I abide by her decision; with no pet sins, what could I say? – A Heart-Song of To-day by Annie Gregg Savigny