Tafsir of Al-Jathiyah 45:19

"Indeed, they will not avail you against Allah in anything"—meaning, in any matter, or in any sense of avail—as you have followed them. The sentence is an independent statement, providing the rationale for the prohibition.

"And indeed, the wrongdoers are allies of one another"—no one allies with them nor follows their whims except one who is a wrongdoer like them.

"And Allah is the Ally of the righteous"—those who take you as their model. So, persist in that upon which you are: specifically, taking Him—Glory be to Him—as your Ally, and turning away from all else, Mighty and Majestic is He, entirely.