Isa 44:3, Ezk 37:14, Joel 2:28-32, Acts 10:43-44
In the Old Testament, God promised to pour out the Holy Spirit on those who obey Him.(Isa 44:3, Ezk 37:14, Joel 2:28-32)
Isa 44:3 For I will pour water upon him that is thirsty, and floods upon the dry ground: I will pour my spirit upon thy seed, and my blessing upon thine offspring:
Ezk 37:14 And shall put my spirit in you, and ye shall live, and I shall place you in your own land: then shall ye know that I the Lord have spoken it, and performed it, saith the Lord.
Joel 2:28 And it shall come to pass afterward, that I will pour out my spirit upon all flesh; and your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions: 29 And also upon the servants and upon the handmaids in those days will I pour out my spirit. 30 And I will shew wonders in the heavens and in the earth, blood, and fire, and pillars of smoke. 31 The sun shall be turned into darkness, and the moon into blood, before the great and terrible day of the Lord come. 32 And it shall come to pass, that whosoever shall call on the name of the Lord shall be delivered: for in mount Zion and in Jerusalem shall be deliverance, as the Lord hath said, and in the remnant whom the Lord shall call.
The Jews believed that keeping the law was obedience to God. But the Holy Spirit did not come upon them, but the Holy Spirit came upon those who believed in Jesus as the Christ. In other words, to obey God is to believe in Jesus as the Christ. (Acts 5:30-32)
Acts 5:30 The God of our fathers raised up Jesus, whom ye slew and hanged on a tree. 31 Him hath God exalted with his right hand to be a Prince and a Saviour, for to give repentance to Israel, and forgiveness of sins. 32 And we are his witnesses of these things; and so is also the Holy Ghost, whom God hath given to them that obey him.
Even the Holy Spirit came upon Gentiles who believed in Jesus as the Christ. (Acts 10:43-44)
Acts 10:43 To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins. 44 While Peter yet spake these words, the Holy Ghost fell on all them which heard the word.