"And that because of false brethren unawares brought in, who came in privily to spy out our liberty that we have in Christ Jesus, that they might bring us into bondage: To whom we gave place by subjection, no, not for an hour: that the truth of the gospel might continue with you." (Galatians 2:4-5)
Paul refers to those who might come into your circle or your church or your life who would also bring with them a strain of thought that issues from and so leads opposite the way of faith, of Christ. While it's one thing to humble yourself and serve others as unto the Lord, it's quite another to be pressed into service through subtle manipulation and that doesn't have the Lord's impetus behind it. That's what Paul's referring to.
One for the road
"This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith? Are ye so foolish? Having begun in the Spirit, are ye now made perfect by the flesh?" (Galatians 3:2-3)
The tenor of Paul's powerful letter to the Galatians is that of keeping out another way of thinking that is in keeping with a way of thinking opposite the way of faith in Christ. To where you believe in God and yet miss the road back He provided. To where you (might) see God and yet endeavor to pave the way with good intentions. It doesn't matter where that road leads, doing the best you can yet ignoring Christ's sacrifice will prevent you from meeting God. When Paul speaks above of "receiv[ing] the Spirit", there's no other way to do this than Jesus.
One for the team
"Have ye suffered so many things in vain? If it be yet in vain. He therefore that ministereth to you the Spirit, and worketh miracles among you, doeth he it by the works of the law, or by the hearing of faith?" (Galatians 3:4-5)
Paul draws the distinction between (I assume) he and whomever came after to try and unravel his teaching and instruction. We need the insight of the Holy Spirit (and also the depth of His forgiveness) in order to see one and the other. Just because someone inspires you doesn't mean they have your best interests at heart. Knowing the what and the why of those who would show you "the way" means already knowing Jesus and reflecting back what they say to Him.