Those who have been baptized into Christ certainly belong to Christ (Gal 3:27-29).
As far as God’s salvation is concerned, baptism and receiving the Holy Spirit are two sides of the same coin. The promise of the Holy Spirit is given to everyone who accepts God’s grace through baptism (Acts 2:38-39). Believers who have been baptized belong to Christ, and the promise of the Holy Spirit is already theirs. In time, they will receive the promise.