It must have been a confusing time to be an apostle. Jesus was talking about leaving, and another Spirit coming after him. We have the benefit of centuries of theologians and church leaders who have helped us come to our knowledge of the Trinity, but this ragtag group of former fishermen and others didn't have that. They were getting it first hand, and it would only really make sense after Jesus was gone. But that slowly formed understanding, that Spirit still lives with us today.