My OED (shorter ) disagrees

The ayes have it on the separation of church and state.
Next item on a busy agenda dismantling of the church and sequestration of property.

Even more fools think socialism is atheistic, when it is as worshiping of authority as any Muslim or Jew

I see there are people here who think this statement is wrong!

You are confusing Secularism with Ecumenicalism, the philosophy of seeking common ground between different view points, when used theologically it seek to bridge differences between all religions and no religion concentrating on shared values rather than quibbling over details
( and before anyone jumps in telling me I am wrong I am talking about Ecumenicalism NOT Ecumenism which is limited to the same idea but limited ONLY to varying Christian Denominations as in the Ecumenical movement of the RC church)

Bullshit!!!

Secular
[ˈsekyələr]
ADJECTIVE
secular (adjective)
denoting attitudes, activities, or other things that have no religious or spiritual basis.Contrasted with sacred.
"secular buildings" · "secular moral theory"
synonyms:
nonreligious · lay · nonchurch · temporal · worldly · earthly · profane · unsanctified · unconsecrated · unhallowed · laic
antonyms:
holy · religious · sacred
christian church
(of clergy) not subject to or bound by religious rule; not belonging to or living in a monastic or other order.Contrasted with regular.
astronomy
of or denoting slow changes in the motion of the sun or planets.
economics
(of a fluctuation or trend) occurring or persisting over an indefinitely long period.
"there is evidence that the slump is not cyclical but secular"
occurring once every century or similarly long period (used especially in reference to celebratory games in ancient Rome).
NOUN
secular (noun) · seculars (plural noun)
a secular priest.
ORIGIN
Middle English: secular (sense 1of the adjective,sense 2of the adjective) from Old French seculer, from Latin saecularis, from saeculum ‘generation, age’, used in Christian Latin to mean ‘the world’ (as opposed to the Church); secular (sense 3of the adjective, sense 4of the adjective, sense 5of the adjective) (early 19th century) from Latin saecularis ‘relating to an age or period’.

Ooooh goody, semantics!
It seems a base definition can be formed, that of worldly: a lay or civil person as opposed to being a member of the clergy.
–
[etymonline.com]
&
[en.wiktionary.org]

sec·u·lar
/ˈsekyələr/
Learn to pronounce
Origin

Middle English: secular (sense 1 of the adjective, sense 2 of the adjective) from Old French seculer, from Latin saecularis, from saeculum ‘generation, age’, used in Christian Latin to mean ‘the world’ (as opposed to the Church); secular (sense 3 of the adjective, sense 4 of the adjective, sense 5 of the adjective) (early 19th century) from Latin saecularis ‘relating to an age or period’.
Definitions from Oxford Languages

The words 'secular' and 'secularism':are two different things.
Secular means 'non-religious.'
Secularism means 'separation of church and state,' and would have to be the law for there to be peace within a country, or amoung nations.
Theocracy--or an imposed secular ideology, for that matter-- is the antithesis of peace, because uniform agreement about ANYTHING is impossible, even between two people.

sec·u·lar
/ˈsekyələr/

adjective
denoting attitudes, activities, or other things that have no religious or spiritual basis.

"secular buildings"

synonyms: nonreligious, lay, nonchurch, temporal, worldly, earthly, profane, unsanctified, unconsecrated, unhallowed, laic
(of clergy) not subject to or bound by religious rule; not belonging to or living in a monastic or other order.

of or denoting slow changes in the motion of the sun or planets.
(of a fluctuation or trend) occurring or persisting over an indefinitely long period.

"there is evidence that the slump is not cyclical but secular"

occurring once every century or similarly long period (used especially in reference to celebratory games in ancient Rome).

Well said.

. . . why not provide source(s) for the claim ?

[secularism.org.uk]

. . .btw, it is not clear-to-me, that secularism is vital for world peace.
I advocate a 'constructed' universal language like Esperanto (there are others).

@Anismail

You are wrong.

Secular (dictionary.com)

adjective

1. of or relating to worldly things or to things that are not regarded as religious, spiritual, or sacred; temporal: secular interests.

2. not pertaining to or connected with religion (opposed to sacred): secular music.

3. (of education, a school, etc.) concerned with nonreligious subjects.

4. (of members of the clergy) not belonging to a religious order; not bound by monastic vows (opposed to regular).

Well! I learned something new. Thanks.