There’s an image used many times to contrast two types of behaviour. In fact this image is used so often that it is hard to know the original source. The context for this image is in the area of moral choices that every Christian faces daily; and the comparison is between a thermometer and a thermostat life. We find that illustrated today in St. Paul’s wonderful advice (Second Reading: Romans 12:1-2). Which teaches, “Do not model yourselves on the behaviour of the world around you, but let your behaviour change, modelled by your new mind. This is the only way to discover the will of God.”
For most of us, there have been many times that we “went along with the crowd” and did whatever the peer pressure of the moment demanded. But this is not the kind of kind of behaviour that Jesus demands from a disciple. It is nothing more than a “thermometer life” – one in which a person responds in time to whatever the sinful social environment around him or her demands. That is what St. Paul meant by “modelling yourself on the behaviour of the world around you.”
On the other hand, a “thermostat life” is one where the person takes positive actions, appropriate decisions based upon truth (i.e., excessive heat or cold). It is a process of simple discernment. This is what St. Paul calls us to do: to recognising the “signs of the times,” the situation at hand, so that we can detect God’s will, rather than act on our selfish desires or those of our peers. Jesus teaches in the gospel today that lack of such discernment can result in gaining the whole world, but suffering the loss of one’s eternal life (Mt 16:26).
The more we renounce our selfishness, the more we walk in the Spirit. The baptised continue to struggle against disordered desires, but will prevail with God’s grace. It is only through prayer that we can detect the will of God and obtain the endurance to do it.
Also see articles ##736, #2520, and #2826in the Catechism of the Catholic Church.