THE GAMER NEXT DOOR

Answer: Mystique got through to him. And his loyalty to Charles overrode any and all false allegiance he had with Apocalypse. Blood being thicker than water and all that. He had all that history with Charles.

Alan Keddie

Answer: He realised the mutants he was helping Apocalypse fight against had always been there for him.

Phaneron ★

Answer: Jean reads minds, so she might read his mind at the first encounter and realised that he was arrogant at the time and not very fond of the place that his brother took him.Also Jean was cold with everyone because of her powers and that every person there afraid of her.

Anastasios Anastasatos

Answer: She's trying to be upfront and friendly, so he knows she's being truthful. That's all.

Chosen answer: Because it's part of his costume.

MasterOfAll

Chosen answer: She only followed him because he claimed to care for mutants. When Angel dies, Storm hears Apocalypse's remark and is insulted. It is then clear to her that Apocalypse doesn't really care about mutants and he is simply using them for their power.

BaconIsMyBFF

Chosen answer: Neither, actually. Apocalypse merely revealed to Magneto a way in which he could use his powers at their current level. Magneto is simply phenomenally powerful.

Phixius ★