The Holy Spirit : Acts 2:1-4 and John 20:21-22
No one can know Jesus as Lord except by the revelation of the Holy Spirit (1 Corinthians 12:3).
All human beings have blood running in their veins. You would not be alive without it. In like manner, you could not be a Christian without the Holy Spirit (John 3:5-8). This is what brings about the salvation experience (John 3:8).
No one was really born again prior to Jesus Christ paying the price of our sins with his death. For it was his death that allowed our sins to be forgiven and the Holy Spirit to be poured out (John 7:37-39; Hebrews 1:9). When Jesus breathed on his disciples in John 20:21-22, they received the Holy Spirit who brings the revelation of salvation.
No one could be saved without the Holy Spirit, not even the disciples. Now their sins were forgiven. They had been born by the Holy Spirit (John 3:8). Notice that here Jesus talked about sins (John 20:23).
The baptism of the Holy Spirit in Acts 2:1-4 is about 'endowing with power for ministry'. Notice that the timid disciples were transformed into bold proclaimers of the gospel.
We all need to seek 'being filled with the Holy Spirit'.