"solid believer" is correct and usable in written English.
You can use it to express someone who is strongly committed to a particular belief or opinion. For example, "She is a solid believer that education is the key to success."
In the small but cohesive Mormon community where he grew up, Hans Mattsson was a solid believer and a pillar of the church.
Bartel is a solid believer and owns 60% of Travelzoo, even after selling 3 million shares so far this year.
I've never been able to apply lipstick in a way that looks natural, and so I'm a solid believer in lip stain.
The signatories described Rouhani as "a politician known to be a firm believer in dialogue and constructive engagement in international relations" who also enjoys "a solid base of support".
Mr. Bicks went to the government job as a rock-solid conservative believer in an unfettered, competitive market.
He's a trim, well-groomed fellow who dresses in Brooks Brothers suits and comes across as a family man, a solid citizen, and a believer in making a good impression, thereby disguising his true intention as a comic performer, which is to pull the rug out from under anyone who happens to be standing on a rug.
Instead of a climate change believer and solid environmental vote in Hillary Clinton, we got the most powerful, most destructive climate denier and anti-environmentalist in the history of the world. .
Still, I am a firm believer that investing in companies with solid business prospects that Wall Street misunderstands, produces superior investment results over time.
But a solid December quarter seems to be making believers of some on Wall Street, as the stock has rallied to all-time highs.
The use of the updated contexts, which he refers to as "surprisingly articulated surfaces", he feels is supported by solid philosophical reasoning: "It is the rationale of the believer," he says.
