Catholics believe that we have an obligation to God to worship him on Sundays. Some Christians or other people might not agree with this obligation. Does this mean that they believe worship is optional? Think about it for a moment: if worship is not obligatory, then somebody could legitimately choose to never worship God. This is unthinkable for a Christian.
Let’s go one step further: if somebody permanently opts out of worshiping God, are they sinning? Matthew, Mark and Luke all show Jesus affirming the first commandment to love God (Matt. 22:37, Mark 12:30, Luke 10:27). Notice the imperative: “You shall love the Lord your God…” It is not optional. How do we show love for him? Through worship. There are other ways as well, but we can’t neglect worship.
Here are some other verses from the New Testament that point to the first commandment (there were more than I thought there would be!):
1 Cor. 16:24:
If anyone has no love for the Lord, let him be accursed.
2 Tim. 3:1-5:
But understand this, that in the last days there will come times of stress. For men will be lovers of self, lovers of money, proud, arrogant, abusive, disobedient to their parents, ungrateful, unholy, inhuman, implacable, slanderers, profligates, fierce, haters of good, treacherous, reckless, swollen with conceit, lovers of pleasure rather than lovers of God, holding the form of religion but denying the power of it. Avoid such people. (emphasis added)
2 Tim. 4:8:
Henceforth there is laid up for me the crown of righteousness, which the Lord, the righteous judge, will award to me on that Day, and not only to me but also to all who have loved his appearing.
Blessed is the man who endures trial, for when he has stood the test he will receive the crown of life which God has promised to those who love him.
Listen, my beloved brethren. Has not God chosen those who are poor in the world to be rich in faith and heirs of the kingdom which he has promised to those who love him?
1 Peter 1:8:
Without having seen him you love him; though you do not now see him you believe in him and rejoice with unutterable and exalted joy.
1 John 2:5:
but whoever keeps his word, in him truly love for God is perfected.
1 John 2:15:
Do not love the world or the things in the world. If any one loves the world, love for the Father is not in him.
1 John 4:21
And this commandment we have from him, that he who loves God should love his brother also.