The other day I was reading Romans and this line jumped out: "If the Spirit of him who raised Jesus from the dead dwells in you, he who raised Christ from the dead will give life to your mortal bodies also through his Spirit that dwells in you" (Rom 8:11).
Paul is underlining that Jesus Christ was raised from the dead through the power of the Holy Spirit. Since Jesus' resurrection is the source of the future resurrection of our bodies, we too will be raised from the dead by the power of the Holy Spirit (at the end of time when Christ comes again in glory.)
But the Church teaches that God anticipated this for Mary. Right after she died, she was taken up into heaven, both body and soul. This is what we celebrate on the Assumption.
Based on Romans, we can say that the Holy Spirit was active in this. And we know from other texts in the New Testament that Mary was filled with the Holy Spirit. When the angel Gabriel asked her consent to become the Mother of God, he said, "The Holy Spirit will come upon you and the power of the Most High will overshadow you." (Lk 1:35) Years later, Mary was present at Pentecost and received the Holy Spirit in an even fuller measure: "They were constantly devoting themselves to prayer, along with Mary, the mother of Jesus" (Acts 1:14).
Mary had a special relationship with the Holy Spirit. She was full of grace, and one effect of grace is the indwelling of the Spirit. So it was extremely fitting that at the close of her earthly life, the Spirit who dwelt in her with such fullness would take her body to heaven as well. Mary received the first fruits of the resurrection of Jesus.
Why is this important to note? Because it shows that the dogma of the Assumption, which Pope Pius XII proclaimed in the Marian year of 1950, has a Scriptural basis. More on that later.