The sacrament of Baptism not only gives us sanctifying grace: it also makes us adopted children of God and heirs of heaven.
We say “adopted” children because God the Father has only one begotten Son—Jesus Christ. He is God’s only Son through generation; the rest of us become God’s children by adoption.
As children of God, we receive our inheritance at the very moment of our adoption, at the very moment of Baptism. Our inheritance is eternal union with God, and we have that inheritance now, once we are baptized.
Nobody can take this inheritance away. Not even God, who has bound Himself by irrevocable promise never to take back what He has given. We ourselves can renounce our rights—as we will do if we commit mortal sin—but no one else can deprive us of our heritage.
The point to be emphasized, and never to be forgotten, is that we are potentially in heaven the moment we are baptized.