Baruch, I think Scripture teaches that the sealing occurs when one puts their faith in Christ, not at Pentecost:
Eph 1:13 in whom also you, hearing the Word of Truth, the gospel of our salvation, in whom also believing, you were sealed with the Holy Spirit of promise,
The disciples were sealed with the Holy Spirit before Pentecost. If they weren't they wouldn't have believed in Jesus. This agrees with Jesus's teachings about being born again. If we are born again, the Spirit has already sealed that person as theirs. No one can say Jesus is Lord except by the Spirit.
Pentecost was a subsequent "pouring out" of the Spirit as prophesied by Joel, and resulted in tongues, prophecy etc..
This pouring out is different to the sealing of the Spirit.
Act 2:16 But this is that which was spoken by the prophet Joel:
Act 2:17 "And it shall be in the last days, says God, I will pour out of My Spirit upon all flesh. And your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams.
Act 2:18 And in those days I will pour out My Spirit upon My slaves and My slave women, and they shall prophesy.
As Barnes says in his commentary:
I will pour out of my Spirit - The expression in Hebrew is, "I will pour out my Spirit." The word "pour" is commonly applied to water or to blood, "to pour it out," or "to shed it," Isa_57:6; to tears, "to pour them out," that is," to weep, etc., Psa_42:4; 1Sa_1:15. It is applied to water, to wine, or to blood, in the New Testament, Mat_9:17; Rev_16:1; Act_22:20, "The blood of thy martyr Stephen was shed." It conveys also the idea of "communicating largely or freely," as water is poured freely from a fountain, Tit_3:5-6, "The renewing of the Holy Spirit, which he shed on us abundantly." Thus, Job_36:27, "They (the clouds) pour down rain according to the vapor thereof"; Isa_44:3, "I will pour water on him that is thirsty"; Isa_45:8, "Let the skies pour down righteousness"; Mal_3:10, "I will pour you out a blessing." It is also applied to fury and anger, when God intends to say that he will not spare, but will signally punish, Psa_69:24; Jer_10:25. It is not infrequently applied to the Spirit, Pro_1:23; Isa_44:3; Zec_12:10. As thus used it means that he will bestow large measures of spiritual influences. As the Spirit renews and sanctifies people, so to pour out the Spirit is to grant freely his influences to renew and sanctify the soul.
As Gill says in his commentary:
By the Spirit is meant the gifts of the Spirit, the spirit of wisdom and knowledge, of understanding the mysteries of the Gospel, of explaining the Scriptures, and of speaking with tongues; and by the pouring of it out, is intended the abundance and great plenty of the gifts and graces of the Spirit bestowed; but yet not all of him, or all his gifts and grace in the large extent of them: therefore it is said, not "my Spirit", but "of my Spirit", or "out of it"; as out of an unfathomable, immeasurable, and inexhaustible fountain and fulness: