({R} magic symbol) ({G} magic symbol) ({W} magic symbol) plus enough mana to pay for commander tax, if applicable available.
Steps
Cast
Marath from the command zone by paying ({R} magic symbol) ({G} magic symbol) ({W} magic symbol) and commander tax, if applicable, causing it to enter the battlefield with a number of +1/+1 counters on it equal to the amount of mana spent to cast it.
When
Marath enters the battlefield,
Mana Echoes triggers, adding ({C} magic symbol) for each Elemental and Beast on the battlefield.
Activate
Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it, creating a 1/1 Elemental creature token.
When the Elemental enters the battlefield,
Mana Echoes triggers, adding ({C} magic symbol) for each Elemental on the battlefield.
Repeat from step 3 until
Marath has one +1/+1 counter on it.
Activate
Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it.
Marath dies as a state-based action due to having zero toughness.
Choose to return
Marath to the command zone from your graveyard.
Resolve the
Marath ability, creating a 1/1 Elemental creature token.
When the Elemental enters the battlefield,
Mana Echoes triggers, adding ({C} magic symbol) for each Elemental on the battlefield.
Activate
Phyrexian Altar by sacrificing three Elementals, adding ({R} magic symbol) ({G} magic symbol) ({W} magic symbol) .
Repeat.
Once you have infinite colorless mana and Elemental tokens, you may cast
Marath, causing it to enter with an arbitrarily large number of +1/+1 counters, and choose
Marath's other modes instead.
Results
Infinite +1/+1 counters.
Infinite colored mana.
Infinite colorless mana.
Infinite commander casts.
Infinite creature tokens.
Infinite damage.
Infinite death triggers.
Infinite ETB.
Infinite LTB.
Infinite sacrifice triggers.
Infinite storm count.
Infinite creature tokens that are infinitely large.