How can a Cleric be proficient in warhammers?

Question

I'm just starting to learn 5e, so I might miss something obvious.

The "Equipment" section of the Cleric description assumes that Cleric can be proficient in warhammers:

Starting Equipment

• a Warhammer (if proficient)

However, there's no Warhammer proficiency in the list:

Starting Proficiencies

Armor: Light Armor, Medium Armor, shields
Weapons: simple Weapons
Tools: none
Skills: Choose two from History, Insight, Medicine, Persuasion, and Religion

Neither I can see one in the "Features" column in the progression table. The only thing about gaining a bonus proficiency is the Life Domain description:

Bonus Proficiency

When you choose this domain at 1st level, you gain proficiency with Heavy Armor.

But it is about Heavy Armor proficiency, not Warhammers.

How can a Cleric get it?

Answer 1

Two cleric domains get proficiency with martial weapons in DnD 5e, and dwarves have a racial proficiency

Cleric Domain

PHB p. 62 and 63 identify Tempest and War Domains as having martial weapon proficiency(for those two domains, using identical language). Other domains do not have proficiency with martial weapons.

Bonus Proficiencies
At 1st level, you gain proficiency with martial weapons and heavy armor.

Race

Likewise, a dwarf cleric of any domain will have racial proficiency with a warhammer.

Dwarven Combat Training
You have proficiency with the battleaxe, handaxe, light hammer*, and warhammer (PHB. p. 20)

Feats

IF the DM allows feats/variant human at first level (optional rule) ... (credit to @nitsua60 for reminding)

A human (variant) cleric could also grab the Weapon Master feat at creation and elect warhammer as one of the four proficiencies.

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*errata change from throwing hammer to light hammer (credit to @MichałPolitowski for reminding of errata change)

Answer 2

Tempest, Twilight, and War Domains also give bonus proficiencies with martial weapons.

Answer 3

Dwarves are proficient in warhammers as a racial feature, so dwarven clerics could start with one.