We pray that through God's grace and mercy, we someday be in perfect unity with God the Father, the Son, and the Holy Spirit in heaven. The path of holiness allows us to experience a hint of that unity here on earth; we get to experience a little bit of what St. Paul said, that Jesus would abide in us and us in him. “Those who love me will keep my word, and my Father will love them, and we will come to them and make our home with them.” John 14:23
I found this prayer by St Elizabeth of the Trinity beautiful.