I keep coming back to the filioque. Might as well try and make sense of it.
The Catholic position is that the Holy Spirit proceeds from both the Father and the Son, and that the procession from the Father is prior. The distinction is subtle but it’s there – and denying it would be heresy. The disagreement between the Orthodox and Catholic views could be seen as a matter of emphasis.
The presence of the filioque in the creed does lend itself to a heretical interpretation on a naive reading but so would its absence. A full theological exposition would be rather wordy so some tradeoff must be made. How much does it matter?
Enter the Reformation.
Many Protestants took the freedom from Catholic authority as a license to interpret theology for themselves, Luther’s intent notwithstanding. They also kept the filioque. The Biblical evidence for the structure of the Trinity can be rather subtle and the filioque is anything but so conditions were ripe for heresy. As such, we’d expect Protestant sects to err in the direction of conflating the processions.
This may be a case of picking your poison. The Reformation without the filioque could well have produced a pattern of denying the divinity of Christ instead.