You can multiclass or even control multiple Class enchantments of the same class. Each Class permanent tracks its own level separately.

If you gain life at the same time a creature is dealt lethal damage, it dies before Cleric Class's level 2 ability can put a +1/+1 counter on it.

An ability that triggers “whenever you gain life” triggers just once for each life gain event, no matter how much life you gain.

Each creature with lifelink dealing combat damage causes a separate life gain event. For example, if two creatures you control with lifelink deal combat damage at the same time, you will gain 2 additional life from Cleric Class's first ability and its level 2 ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), you will only gain 1 additional life and the ability will trigger only once.

If you gain an amount of life “for each” of something, that life is gained as one event and the ability will trigger only once.

When Cleric Class's level 3 ability resolves, you gain life equal to the creature's toughness as it exists on battlefield, which may be different than its toughness in the graveyard.

Each Class has five abilities. The three in the major sections of its text box are class abilities. Class abilities can be static, activated, or triggered abilities. The other two are level abilities, one activated ability to advance the Class to level 2 and another to advance the Class to level 3.

Each Class starts with only the first of three class abilities. As the first level ability resolves, the Class becomes level 2 and gains the second class ability. As the second level ability resolves, the Class becomes level 3 and gains the third class ability.

Gaining a level won't remove abilities that a Class had at a previous level.

Gaining a level is a normal activated ability. It uses the stack and can be responded to.

You can't activate the first level ability of a Class unless that Class is level 1. Similarly, you can't activate the second level ability of a Class unless that Class is level 2.

Some Class cards have an effect that increases when more are under your control. For example, if you have multiple Barbarian Class cards, you roll that many additional dice and ignore that many of the lowest rolls.