One day a certain person came and told Jesus that his mother and brethren were outside desiring to speak with Him. To that person Jesus said....

":....Who is my mother? and who are my brethren?
And he stretched forth his hand toward his disciples, and said, 'Behold my mother and my brethren'!
For whosoever shall do the will of my Father which is in heaven, the same is my brother, and sister, and mother. "(Matthew 12:48-50)

The KEY here is seen in the doing of the will of God! He is using the circumstances to state that anyone found outside of the will of God is in no way related to Him. Therefore it becomes imperative to know exactly what Jesus says and means in "DOING" the will of His Father. For this; as always, we go to the Scriptures to find the answer....

Then said they unto him, What shall we do, that we might work the works of God?
Jesus answered and said unto them, 'This is the work of God, that ye believe on him whom he hath sent. (John 6:28-29)

The WORK of saving us is Finished! "Believing on Him", IS the will of God!

It is NOT the will of God to believe on Him + religion;
or believing on Him + your doing other saving/good works in order to help save yourself;
or believing on Him + taking part in sacraments;
or believing on Him + stand on your head and whistle Dixie while twirling plates and saucers in each hand, balancing a chair on your chin and spinning a table with your feet, all at the same time;
or believing on Him + anything else.

THE WORK OF SAVING US IS FINISHED! BUT... only those who believe on Him alone + Nothing are considered to be one of "His Family" and will be saved!!! This is the will of God: To believe on His Son alone!

That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. (Romans 10:9-10)

Believing upon Christ means just that: "Believing!" - An all out belief, believing that He alone was sent to pay for our sins (and He did so by dying on the cross for our sins and did it all without our help), and Believing that God has raised Him from the dead so that we who believe may receive the righteousness that saves us, the righteousness that comes by faith alone in believing on Christ alone!

I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain. (Galatians 2:21)

To "frustrate" means to make "null and void". By the Spirit Paul is saying that if the kind of righteousness that saves us ever came by the law, by our doing the law, or by our keeping the law, or by our being religious (going to church, etc.) in order that we may try and keep the law, then for us "Christ" has died in vain!!! His death is being as "worthless" to all who claim to be of the above mind-set. Again Paul says....

For if they which are of the law be heirs, faith is made void, and the promise made of none effect: (Romans 4:14)

There is no way in heaven or in hell that our obeying the law in order to save us (i.e. through our going to church, keeping and doing church dogma, traditions, or even our trying to keep the Holy Law itself) will gain us heaven. In fact just the opposite is true:
Because the law worketh wrath: for where no law is, there is no transgression. (Romans 4:15)

If we are trying to save ourselves through keeping the law, then all that our trying to keep the law will do for us is to incur God's wrath! Why? Because it proves that such "doings" as our trying to keep the law in order to save us, or help save us, or help keep us saved once we are saved, only proves beyond a doubt our unbelief!

For they being ignorant of God's righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God.
For Christ is the end of the law for righteousness to every one that believeth. Romans 10:3-4)

Here is the difference between GRACE and LAW: Grace is given without cost on God's part, earned by Christ alone and given freely only to those who believe on Christ apart from works! The Law depends on our part in keeping it, thus it is connected to us whenever we try to "earn" our own salvation, which clearly reveals: "Unbelief" in His Son and His already finished work.

There are no additions to be added to Christ and no additions or subtractions to His already finished work! The GOOD NEWS is that the work of saving us is OVER... IF we believe on Him alone, thus come and lay hold of Him by that very faith.

For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.
For if they which are of the law be heirs, faith is made void, and the promise made of none effect... (Romans 4:13-14)

Therefore all that God will recognize from us is "Faith", the kind that hears, repents, drops everything, comes-a-running and lays hold of Jesus. Anything short of this stems because of "Unbelief"!

Once the person becomes "Regenerated/Saved" (i.e. "Born Again") because of such Faith, then, and only then he receives Power (i,e, "The Holy Spirit") that enables him to live the Christian life and establish the law of God in his heart.
Does this mean that we do away with the law of God because we now have "Faith"? No way:, instead we have received Power and are now both willing and able to obey the law of God. Paul says it like this...

Do we then make void the law through faith? God forbid:, yea, we establish the law. (Romans 3:31)

Therefore we "Establish" the law because we are saved and know it, not the other way around. The difference is night and day.

How does one "Keep" his word? How does one obey the law of His word?
One Example is: When the person hears the Spirit of Christ telling them to "Forgive a certain person they had always considered their enemy, and in turn do good to that person who persecuted them", then that is exactly what they go and do.
They now become a peacemaker and forgive them from the heart and make restitutions (if necessary) to one age-old "enemy" at a time until they have gone through them all, and in so doing they will have helped God to cleanse their heart of every evil thought and way through their obedience.

But whosoever keepeth his word,in him verily is the love of God perfected: hereby, know we that we are in him. (1 John 2:5)