It was a baptism of fire.

Meaning

It was a challenging and difficult experience that served as a steep learning curve.

Examples

• John: My first week at the new job was incredibly intense.
• Alice: Oh really? How so?
• John: <b>It was a baptism of fire</b>. I was thrown straight into the deep end.
• Alice: I can imagine! Yeah, starting a new job can be like that, a real <b>baptism of fire</b>.