"So I say to you: Ask, and it will be given to you; search, and you will find; knock and the door will be opened to you. For everyone who asks receives, everyone who searches finds; everyone who knocks will have the door opened. What father among you, if his son asked for a fish, would hand him a snake? Or if he asked for an egg, hand him a scorpion? If you then, evil as you are, know how to give your children what is good, how much more will the heavenly Father give the Holy Spirit to those who ask him." (Luke 11: 9 – 13)
Condition for receiving the Holy Spirit: desire and prayer – "Ask and you shall receive."
"Jesus Christ himself is the "Amen." He is the definitive "Amen" of the Father's love for us. He takes up and completes our "Amen" to the Father: "For all the promises of God find their Yes in him. That is why we utter the Amen through him, to the glory of God." (Catechism of the Catholic Church # 1065)
When we ask God with faith for the Holy Spirit, we receive Him. St. Augustine writes: "May your Creed be for you as a mirror. Look at yourself in it, to see if you believe everything you say you believe. And rejoice in your faith each day."