How can we know the invisible things God did when we believed?
We know them by faith, because the book says so. They are invisible, untouchable acts of God, such as the Holy Spirit baptizing every believer into the body of Christ when they believed. We cannot feel or see it, but we believe it happened.
Les explains that Paul writes only to believers, so when he says "by one Spirit are we all baptized into one body," the "all" means every believer, not everyone. This baptism is not into a local church or denomination but into the body of Christ. It is an act of God that we cannot put our hands on; it is in the area of the invisible. Les reminds us that back in Romans he listed all the things God did the moment we believed, and every one of them is such that you cannot put your fingers on it, see it, or touch it. We know they happened by faith, because the book says so. He gives the example of Spirit baptism: I cannot look back and say I remember or feel when the Holy Spirit put me into the body, nor do I have proof or witnesses, but I know it happened because the book says it did. The moment every child of God believed, the Holy Spirit baptized them into the body of Christ.