On one level today's first reading deals with the gift of eternal life which Christ won for us by his incarnation, suffering, death, and resurrection. As St. Paul reminds us, this eternal life is not just for our souls but for our bodies.
If the Spirit of the one who raised Jesus from the dead dwells in you, the one who raised Christ from the dead will give life to your mortal bodies also, through his Spirit that dwells in you.
And so in the Apostles Creed we proclaim the resurrection of the body.
On another level, St. Paul challenges all of us who have through our baptism (and confirmation) received the gift of the Holy Spirit to live in that Spirit. St. Paul is not hear making the argument that the flesh is in itself evil. It is a part of God's creation and as cited above will be redeemed. What he is warning against is allowing the flesh to be the driving force.
When we live in the Spirit we have our priorities in order. Even for us as priests this is hard to maintain. If we go back to the scriptures and the teaching of the Chruch, my primary work as a priest is prayer, especially the Liturgy of the Hours and the celebration of the Eucharist. Yet I too fall into the trap of thinking that constantly responding to phone calls, text messages, email, constant meetings, etc is the real measure of being a good priest.
Each year how many marriages do I see fail because one or both parties is living in the flesh, letting their job become their God, and one true love? Even when they are together they are not fully present to one another.
The person who lives in the Spirit never forgets that this material world will all pass away. The two great commandments remind us that God and those who are ultimately welcomed into heaven are eternal. Eternal life is the only real long-term investment. The person who lives in the Spirit never forgets this.
Truth is, we all vacillate. We need things like crucifixes, images, and prayer like the rosary to call us back, refocus us.