Friends, our readings today could seem confusing at first.
First, we hear Moses tell his people “I set before you... a blessing for obeying the commandments of the Lord, your God..; a curse if you do not obey the commandments of the LORD, your God..." (Dt 11:18,26-28,32)
In contrast, Paul tells the Romans that "...the righteousness of God has been manifested apart from the law,... the righteousness of God through faith in Jesus Christ... a person is justified by faith apart from works of the law." (Rom 3:21-25, 28)
On the one hand, we are to obey the commandments - to do what God commands. On the other hand, we are justified by faith, not but what we do. Could Moses and Paul be contradicting each other? (Paul was a student of Rabbi Gamaliel, well-versed in Scripture; he would certainly be familiar with Moses' words from Deuteronomy.)
In the Gospel reading, Jesus steps in and clears up the seeming contradiction. He warns that "Many will say to me... 'Did we not drive out demons in your name? Did we not do mighty deeds in your name?' Then I will declare to them solemnly, 'I never knew you...'" He continues that "...everyone who listens to these words of mine but does not act on them will be like a fool who built his house on sand." (Matt 7:21-27)
We can do mighty deeds, but if Jesus doesn't know us, then our deeds come to nothing. We cannot just act like Christians, we must really be believing Christians, as Paul said.
We can listen to Christ's words, go to Mass each week, and really believe in God. If we do not act on that belief - if being a Christian doesn't change us - our belief comes to nothing. We must truly live as Christians and follow God's laws, as Moses said.
There is no contradiction because both Moses and Paul are correct. We are not saved by faith alone; faith without works is dead (James 2:20). And we are not saved by works alone; those works must be built on a foundation of real faith - a house built on the Rock instead of shifting sand.
If each of us is to be a temple of the Holy Spirit, a flesh and blood house of God, we must mind both the foundation and the structure of that house. It must be built of good works and grounded on faith.