Our own minds, even when seeking spiritual truths, can lead us astray if they aren't submitted to God's Word.
Paul calls out the 'fleshly mind' that leads to speculation and pride. This refers to our natural human reasoning, which, apart from the Spirit's illumination, is self-centered and prone to error. The false teachers were 'puffed up without reason' because they were delving into hidden, unseen things ('intruding into those things which he hath not seen') based on their own flawed logic and supposed visions, rather than on clear revelation.
This is a timeless warning. It's easy to get caught up in complex spiritual theories, 'mysteries,' or visions that seem profound but lack a solid foundation in Scripture. When our thinking isn't grounded in God's revealed truth, it can lead to arrogance and a distorted view of reality, making us vulnerable to deception.