Karl Barth reflects on the importance of the preposition in the Apostles’ Creed:

I believe in, credo in, means that I am not alone. In our glory and in our misery we men are not alone. God comes to meet us and as our Lord and Master He comes to our aid. We live and act and suffer, in good and in bad days, in our perversity and in our rightness, in this confrontation with God. I am not alone, but God meets me; one way or another, I am in all circumstances in company with him. That is, I believe in God, the Father, the Son, and the Holy Spirit.