Strive to enter

Hebrews 3:12-13
Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin.

"an evil, unbelieving heart" makes you depart from God.

What would a "believing" heart look like?

It would look like Jesus' mindset toward the Father: total trust in the Father's care and character and wisdom; total submission to His will; total faith in His Word being true and trustworthy and faithful.
It would mean never doubting the Word to be true, or the promises to fail, or the threats to be empty.
Never questioning the Father's clear will, nor rebelling against it, nor fighting His providence.
Never seeking to rule myself, but being yielded up to the Father unreservedly.
It would mean complete submission to the Father, devotedly, unquestioningly.

Real faith believes, submits, obeys.
Satan would deceive us, making sin and self-rule look attractive and making submission to the Father a repulsive choice.  The deceitfulness of sin can harden your heart toward God, such that you mock His rule, ignore His Word, and run toward destruction as though that were life.

How can we exhort one another against the deceitfulness of sin?
We must be willing to bring God's Word to bear on one another's lives. Which means WE must know the Word enough to do that. We must be willing to be salt and light to one another, not just smiling and nodding and having happy social times. We must be willing to call out one another's attitudes as being incongruent with imitators of Christ.

Hebrews 3:18 - 4:1
And to whom did he swear that they would not enter his rest, but to those who were disobedient? So we see that they were unable to enter because of unbelief.
Therefore, while the promise of entering his rest still stands, let us fear lest any of you should seem to have failed to reach it.

Hebrews 3:12-13
Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin.

Hebrews 4:11-13
Let us therefore strive to enter that rest, so that no one may fall by the same sort of disobedience. For the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul and of spirit, of joints and of marrow, and discerning the thoughts and intentions of the heart. And no creature is hidden from his sight, but all are naked and exposed to the eyes of him to whom we must give account.

https://www.desiringgod.org/messages/the-word-of-god-living-active-sharp

Those three passages are tied together in this way:
It is unbelief that causes us to fail to reach Heaven.
It is the deceitfulness of sin that hardens us.
It is the Word of God that can pierce the heart and expose the unbelief lodging there, and show sin's deceit for what it is.

So to help one another make it safely Home, we must wield the Word of God with one another - exhorting daily! - applying it to each other's lives and hearts and attitudes and words, that we might be a prod to get others back on the right Path when sin lures them to wander off it.

This is why studying the Word itself is so important.  It is not devotionals that will pierce our hearts and expose our sin - it is the Word itself.
This is why memorizing and meditating on the Word is so important.  It is a tool the Spirit can use to keep US from being deceived by sin, and it is a tool that we need to help others get safely to Heaven.