Can I use two conjunctions in one sentence?

I learned that if I use two conjunctions in one sentence, I need at least three separated sentences.
(subject+verb conjunction subject+verb conjunction subject+verb.)

For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.

In this sentence, the two conjunctions ‘for’ and ‘but’ are used without main(or pivot??) sentence.

Then, why is this sentence grammatical?