"You turned to God from idols, to serve the living and true God."
When St. Paul wrote today's epistle to the Thessalonians, they had fairly recently converted from paganism to Christianity. In fact, some of their fellow citizens were still pagans and worshipped the idols of their false gods. It is quite likely that Paul mentioned this in order to confirm them in their resolve not to return to pagan practices. He was concerned that the bad influence of the pagans in their midst might cause them to lose their newly acquired Catholic Faith.
In the modern world we don't see much of the idol worshipping of centuries ago, but idolatry of a sort is still something of a problem for us to be concerned with, for we have modern pagans in our midst who might prove a temptation like Thessalonians of old. It is useful for us to review what is prohibited by the First Commandment, so that we can be on our guard against such temptations.
"I am the Lord thy God, thou shalt not have any false god before Me."
Obviously, this prohibits us from worshipping any god other than the one true God. One can't be, at the same time, a Catholic and a Hindu, or a Catholic and a Moslem. The Psalms tell us that "the gods of the gentiles are devils." (Ps. 95) That is to say that any power the false gods may exercise must come from the devil since it does not come from God.
We are also enjoined by the First Commandment to worship in the way God revealed to us that He wants to be worshipped. That is to say that when God became man he gave us the Mass and Sacraments as the means of worshipping Him. In doing so, He replaced the animal sacrifices of the Jews with the Sacrifice of His Son on the Cross, which we renew each time Mass is celebrated. Consequently it would be wrong for us to participate in the religious ceremonies of the Jews, or in those of Christians who deny the reality of the Mass. Doing so would be going counter to what Jesus Himself instructed us to do.
There is a danger for some of us that we might idolatry in the things around us. How many of us spend more time watching television than in prayer? Or in some other unnecessary activity?
We ought not to put any faith in superstitious practices; consulting mediums or fortune tellers, having our palms read, trying to predict the future from our dreams, or carrying "lucky" charms. Again, to the degree that any of these things actually work, they work through the power of the devil.
We must be on our guard in the way we use even the sacramentals of the Church. It would be seriously wrong to attribute magical powers to things like holy water, or to a crucifix, or a medal or a scapular. Likewise it would be a sin against the virtue of hope to think that our soul will be saved by wearing or carrying any religious object while living an unholy life.
It can be a positive good to have pictures and statues of our Lord and the Saints. But, again, these must be treated as reminders of the holy people they represent, and not as objects of worship. (Some Catholics, by the way, do worship statues—but they shouldn't.) And just like the sacramentals, no number of statues or pictures will bring about our salvation if we don't live as good Christians.
Holy things must be used in a holy way. To do otherwise is the sin of sacrilege. We have a positive obligation to see stop others from committing sacrilege if we are able to do so. This duty is proportional to the holiness of the thing in question. For example, misusing a consecrated chalice is more seriously wrong than misusing a picture of a saint. Both are minor relative to abuses against the Blessed Sacrament Itself. If we are unable to stop those who commit sacrilege, we must at least disassociate ourselves from them, lest we seem to condone their actions.
Finally, holy things must not be bought and sold. It is a serious abuse to sell religious articles that have been blessed or consecrated; and even more serious a sin to charge for the administration of the Sacraments. This is called simony, after Simon Magus, who tried to buy the ability to Confirm from St. Peter.
Now, I know that I have mentioned a lot of negatives: Don't worship false gods; don't worship the true God in a false way; don't engage in superstitious practices; don't treat the sacramentals as though they were magical; don't condone sacrilege; don't buy and sell holy things. But really, we ought to view the First Commandment in a positive light. It is telling us after all, that we should love and worship the one true God with all of our heart and mind and soul--and not just for an hour on Sundays--but at all times and in all places. In the Mass certainly, as often as we are able; in Holy Communion, again as often as we can; in keeping the Commandments; in prayer and in fasting; in all of our good works; literally, in everything we do.