Without faith it is impossible to please God. And as scripture reveals the reality of being born again involves steps of faith that have nothing to do with the law. Your point concerning the need to walk in the Spirit in order not to sin is without a doubt true. However, one must first have the indwelling presence of the Spirit before that is even possible. Acts 19:1-7 has nothing to do with walking in the Spirit, it provides a window into the NT rebirth (being born again). The elements presented confirm what all other conversion accounts do. They involve belief, obedience to water baptism in the name of Jesus, and evidence that the individuals received the indwelling presence of the Holy Ghost. (Acts 2:38-41, 8:12-18, 9:17-18, 10:43-48, 19:1-6, 22:16)
It is obvious why Paul asked the Ephesians the question. Paul knew that belief and receiving the Holy Ghost were essential elements of salvation. (Rom. 8:9) Note that the Holy Ghost DID NOT came to dwell in those individuals at the point of their believing in Jesus, nor when they were rebaptized in water in the name of the Lord Jesus. It was when Paul laid hands on each of the twelve. Also relevant is the fact that elsewhere in scripture individuals were indwelt by the Holy Ghost after believing in Jesus, and before obedience to the command to be water baptized. However, one thing that is consistent is there was indisputable evidence that accompanied the experience.
Lastly, consider what Jesus Himself said about receiving the Holy Ghost. His words speak directly to the error that receiving the Holy Ghost happens spontaneously when a person believes: "If ye then, being evil, know how to give good gifts unto your children: how much more shall your heavenly Father give the Holy Spirit to them that ask him?" (Luke 11:13)