Guardian Idol — ruling 1

If Guardian Idol has any +1/+1 counters on it, those counters will remain on it after it stops being a creature. Those counters will have no effect as long as Guardian Idol isn't a creature, but they will apply again if it later becomes a creature.

Guardian Idol — ruling 2

Activating the last ability of Guardian Idol while it's already a creature will override any effects that set its power or toughness to a specific value. Effects that modify power or toughness without setting them to a specific value will continue to apply.

Guardian Idol — ruling 3

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Guardian Idol : règles, fonctionnement et interactions (MTG)

Guardian Idol is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This artifact enters tapped. : Add . : This artifact becomes a 2/2 Golem artifact creature until end of turn.

How it works : Guardian Idol

Original English text — Wizards of the Coast / Scryfall

#1November 17, 2017· Wizards of the Coast
If Guardian Idol has any +1/+1 counters on it, those counters will remain on it after it stops being a creature. Those counters will have no effect as long as Guardian Idol isn't a creature, but they will apply again if it later becomes a creature.
No other card is named in this ruling.
#2November 17, 2017· Wizards of the Coast
Activating the last ability of Guardian Idol while it's already a creature will override any effects that set its power or toughness to a specific value. Effects that modify power or toughness without setting them to a specific value will continue to apply.
No other card is named in this ruling.
#3August 1, 2008· Wizards of the Coast
A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
No other card is named in this ruling.