Someone showed me a verse in the Bible saying the Spirit of God being in all men. How can this be?
The passage you are referring to is:
"And it shall come to pass in the last days, says God, That I will pour out of My Spirit on all flesh; Your sons and your daughters shall prophesy, Your young men shall see visions, Your old men shall dream dreams" (Acts 2:17).
While "all flesh" does mean all people, it is being qualified by the list that comes immediately after the phrase. It means all types of people: men, women, young, and old. It doesn't imply that the Spirit would come upon every individual in the world. If that were true, then Peter's later promise:
"Then Peter said to them, "Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit" (Acts 2:38).
If people already had the Spirit, there would be no need to promise them that they could receive the gift of the Spirit.