I am made right with God
only by true faith in Jesus Christ.
Even though my conscience accuses me
of having grievously sinned against all God’s commandments
and of never having kept any of them,
and even though I am still inclined toward all evil,
nevertheless,
without my deserving it at all,
out of sheer grace,
God grants and credits to me
the perfect satisfaction, righteousness, and holiness of Christ,
as if I had never sinned nor been a sinner,
as if I had been as perfectly obedient
as Christ was obedient for me.
All I need to do
is to accept this gift of God with a believing heart.
When I say that by faith alone I am right with God;
it is not because of any value my faith has
that God is pleased with me.
Only Christ’s satisfaction, righteousness, and holiness
make me right with God.
And I can receive this righteousness and make it mine
in no other way than by faith alone.

—from Heidelberg Catechism, Q&A’s 60-61

And how do I accept this gift with a believing heart?

By the regenerating work of the Spirit.  Thanks be to God for His indescribable gift!