It is commanded in the Scriptures by St. Paul:
Hebrews 10
23Let us hold fast the profession of our faith without wavering; (for he is faithful that promised

24And let us consider one another to provoke unto love and to good works:
** 25Not forsaking the assembling of ourselves** together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.
** 26For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,
**
***If we forsake the assembly, we sin wilfully.
27But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.
28He that despised Moses’ law died without mercy under two or three witnesses:
He compares forsaking the assembly to the person who despised the law of Moses and is stoned without mercy. A mortal sin.
29Of how much sorer punishment, suppose ye, shall he be thought worthy, **who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? **
30For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people.
31It is a fearful thing to fall into the hands of the living God.
***It is in the Mass that we receive the Body of Christ and the Blood of the Covenant in the Eucharist. Therefore if we forsake the Mass, we have trod under the Son of God and counted the Blood of Christ by which we are sanctified an unholy thing.
I think that constitutes a mortal sin.***
But please, tell me what you think.