How did you receive the Spirit?
Gal 3:2-5 I would like to learn just one thing from you: Did you receive the Spirit by observing the law, or by believing what you heard? Are you so foolish? After beginning with the Spirit, are you now trying to attain your goal by human effort? Have you suffered so much for nothing— if it really was for nothing? Does God give you his Spirit and work miracles among you because you observe the law, or because you believe what you heard?
Paul presumes that they had received the Spirit. Afterall it says, "if anyone does not have the Spirit of Christ, he does not belong to Christ." Rom 8:9 Once a person comes to faith in Christ he receives the Spirit. The point here is that the receiving of the Spirit is not a matter of ceremony or following certain rules and regulations. It's not even a matter of prayer, nor of wishful thinking. It's a matter of believing the gospel.
Paul says elsewhere, "As you have therefore received Christ Jesus the Lord, so walk in Him." Col 2:6 The way you're supposed to receive Christ is by faith and not by ceremony; not by following certain rules and regulations. And in the same manner you received Christ, so you're supposed to live - by faith.
By "human effort" Paul is referring to relying upon rituals to attain the goal. The observance of days, the eating or abstaining from certain foods, or even water baptism and communion - if people are relying on such things to achieve their goal, then they are caught up in legalism. It is not that rituals are bad, or even inappropriate. For some, such as water baptism and communion, are even commanded by the Lord. But the issue of legalism comes up when people rely upon the ceremonial aspects of these to confer some sort of grace or salvation.
Furthermore when Paul speaks of "faith" here obviously he is referring to faith in Christ and not faith in ceremonies or rituals. For indeed the Galatians were exercising "a faith" - but it was a faith in ceremonies and not a faith in Christ.