The Isaac community is passionate, and suggesting "cheating" can start a flame war. However, there are legitimate, non-malicious reasons to use a save editor.

Warning: Manual editing without knowing offsets/formats is likely to corrupt save.

A save editor is a third-party tool (not made by Edmund McMillen or Nicalis) that allows you to directly modify your repentance.save file. Instead of grinding Greedier Mode for 50 hours, you can toggle achievements, add items, unlock characters, or even resurrect a dead save file.

The most popular tool for this is Isaac Save Editor by Hyphen-ated (often called the "web-based save editor" or the standalone app).