Many people view the Kingdom of Heaven as a destination or goal, that we achieve *after* we die. But the historical Christian perspective, is the Kingdom of Heaven is something we achieve right here, right now, on Earth; inside of us. And it's like a seed that is planted inside of us, that we have to nurture and grow, and we die to ourselves, and let God live within us — until we bear fruit. And we either belong to God, or we belong to the Devil. And when we die, God simply claims what is already his, or if we belong to the Devil, the devil/demons simply claim what is already theirs; as spoken of in Romans 8:9-11:
So we can see whoever does not have the Spirit of God/Christ dwelling within them, does not belong to God. And if we have the Spirit of God that dwells inside of us when we die, the same Spirit that already dwells inside of us will raise us up into Heaven. And if we don't have the Spirit of God dwelling inside of us, our soul is dead to sin and by default we go to Hell. And here we can see Christ talks about how if we keep Christ's/God's commandments and are obedient to his word, Christ (God the Son) and God the Father will come in and dwell within us, as spoken of in John 14:23:
So the secret to Eternal life is having the Spirt of God dwell within us: The Father, Son, and Holy Spirit. Once we have our sins forgiven, the Holy Trinity comes in and dwells within us, and sanctifies us, and we return to a state of Innocence and Holiness. This is what we Catholics refer to as being in a "State of Sanctifying Grace" or a being in a State of Grace for short. So this is how we know *when* our sins are forgiven is when the Spirit of God comes in and dwells within us. And we can either be in a State of Grace, or in a State of Sin, just like how we go through life moving between various states: Infancy, Childhood, Adult, and Elderly. And the gift of Sanctifying grace is a free gift, but in order to retain the Spirit of God, we have to cooperate with the Spirit of God by being obedient and staying out of sin and keeping the commandments, and doing good works of love and charity; otherwise we're being rebellious and disobedient, and the Spirit of God will leave us. As Christ says in Matthew 7:21:
So Sanctifying Grace is a free gift, but even Halloween candy is free but we still have to put in a little bit of effort and go and get it (Be Baptized or Confess and Repent of our sins). So us Catholics agree with many Protestants that the initial Justification is free, but we have to do good works and remain obedient to retain the Spirit of God dwelling within us; I.e. "the one who does the will of my Father in heaven." And a frequent Protestant misunderstanding is that Catholics believe we can earn our way into Heaven, which Protestants have it backwards. First we get the Grace which is free, then we can do good works of love pleasing to God, which is simply us cooperating with Grace I.e. the Will of God. If we're not in a State of Grace, our good works are *not* pleasing to God because we'll have ulterior motives, as spoken of in Isaiah 64:6:
But if we're in a State of Grace , we're sinless (or at least free from mortal/serious sin) and we'll be doing good works purely out of selfless love, because the Spirit of God dwelling within us makes us want to do good works purely out of love without worrying about any gain. And thus we do good works of love that are pleasing to God. And Protestants often confuse Works of the Law mentioned in Romans and Galatians, with Works of Love. And the Works of the Law mentioned in Romans and Galatians are the Jewish Works of the Law: Dietary laws, Circumcision, Sabbath Laws, and the 613 laws in the Old Testament. But back to being in a State of Grace and being selfless; the more we die to ourselves, the more we let Christ live, as St. Paul said in Galatians 2:20:
So we can see St. Paul increasingly died to his selfish desires, and let Christ live I.e. the Spirit of Christ/God lived within him, and Christ/God acted through St. Paul. And the more we die to ourselves and the more selfless we become; and to achieve perfection like Christ, we give more of our possessions away. So the more selfless we are, the more we are like Christ, and the more the Spirit of God dwells within us, I.e. the Kingdom of God dwelling with us. This matches Christ's parable of the jewel hidden in the field; how the man discovered the Kingdom of Heaven and sold everything he had, so he could have the Spirit of God dwell within him by being selfless, in Matthew 13:44-46:
And this is my own experience as well. Now I haven't sold all my possessions, but I have given up almost all sinful/selfish pleasures to remain in a State of Grace, matching Christ's parable of the hidden jewel. And every Lent I'm giving up another selfish desire and dying to myself more and more to increase the amount of Grace I have.
So how do we receive the Spirit of God and enter into a State of Grace? By being Baptized, which washes away our sins, as spoken of in Acts 2:38:
We also see this through Christ's own Baptism, how the Holy Spirit descended upon him at his Baptism, and God the Father spoke: "This is my beloved Son, in whom I Am well pleased." Likewise at our own Baptism, the Holy Spirit descends upon us, and we become adopted sons and daughters of God the Father, and Sanctifying Grace is infused into our souls. Except at Christ's Baptism, Christ made the water holy for our sake. And if we commit a mortal/serious sin after our Baptism, our soul goes into a State of Sin, and we have to go to Confession, and the sin is wiped clean off our soul and we return to a State of Grace. So in John 14:23 above, we can see Christ says he and his Father will dwell in those who keep his word. And in many other verses, we can see the Holy Spirit dwells within us as well, so all three persons of the Holy Trinity dwell within us, as spoken of in 1 Corinthians 6:19:
And Christ also spoke of his own body as a "Temple" and how he would destroy it and rebuild it in three days. And we know in the Old Testament days, the Temple was the dwelling place of God. Now after Christ's pure sacrifice, our own bodies are Temples which are the dwelling places of God. And for God to remain in our Temples, they have to remain pure. Because God only dwells within pure, sinless, and holy temples. And surprisingly, the secret to Eternal life is wholly contained in the "Hail Mary" prayer, which is 100% biblical: "Hail Mary, full of Grace, the lord is with you" [Luke 1:28] Blessed are you among women, and blessed is the fruit of your womb [Luke 1:42]." So we can see from these verses the Blessed Virgin was full of Grace, proving she was sinless and pure, and the Spirit of God was dwelling within her. So the secret to salvation and eternal life is right there in one of the most well known prayers hiding in plain sight, yet most people don't know about it! And one more thing we can see from the previous verse, is that we all are like trees producing fruit, or rotten trees producing no fruit; while the Blessed Virgin's fruit she produced "fruit of her womb" was Christ himself. Now God is Eternal, but yes the Blessed Virgin is the Mother of God's human nature; Christ is a divine person, but he has both a human nature and a divine nature.
Another biblical verse that Christ touched upon was how we must become like little children, or we'll never enter the Kingdom of Heaven; mentioned in Matthew 18:3:
And this is exactly what happens when we're in a State of Grace; we return to a state of Innocence. Like little children are initially innocent, but then become more and more corrupt over time. Like little children don't have sexual thoughts, don't hold grudges, aren't bitter and mean, they aren't cold-hearted; they don't worry, they aren't full of anxiety; they are innocent and pure. And when the Spirit of God comes in and dwells within us, we return to this state of Innocence. Like me personally I used to watch tv shows with sex and cursing, now I can't stand stuff like that ever since the Spirit of God came and dwelled within me. Because when in a State of Grace, we feel what God feels, and behavior like cursing, is disgusting. But adults become desensitized to these behaviors over time, but children aren't. Like think about if someone curses around a child, the child is like: "Oh they said a bad word!" Children instinctively know it's wrong and are repelled by it. And when in a State of Grace, we are repelled by cursing as well. I.e. We're returned to a child-like state of Innocence. And a good way to test if you're in a State of Grace, if you're doing things and watching things and reading things, that a child would be comfortable around you doing those things that you're doing.
And along with being in a state of Innocence that we are in when in a state of Grace, we return to a state of Holiness, and sin becomes disgusting. Since the Spirit of God dwells within us, we feel what God feels, and we feel the disgust that God has for sin. And this state of Holiness matches Hebrews 12:14:
So in order to see the Lord in Heaven, we have to achieve Holiness, and being in a state of Grace we return to that Holiness. And we can see that nothing unclean shall enter Heaven, as spoken of in Revelation 21:27:
And along with holiness, comes a need to turn away from Sin. And the Spirit of God dwelling within us, assists us in overcoming addictions and other sinful behaviors; since we feel what God feels, sin all of a sudden becomes disgusting. I.e. Sanctifying Grace literally Sanctifies us I.e. It makes us Holy. And if we commit serious(mortal) sin, the Spirit of God will leave us and we'll return to a state of Sin. And the Bible warns us not to be deceived, that sinners will not inherit the Kingdom of God, neither within us or the world to come, as spoken of in 1 Corinthians 6:9-11:
And one of Christ's parables I wanted to touch upon and how it logically matches the concept of being in a State of Grace, is the parable of the Bridesmaids awaiting the Bridegroom, and the Bridesmaids who weren't ready when the Bridegroom arrived, were left behind; in Matthew 25:1-13:
So we can see here, those who are in a State of Sin when they die, will be left behind when they are called back to the Father's house; as they weren't ready and weren't prepared. While those in a State of Grace were ready and prepared for when they died, they entered with the Bridegroom to the Wedding Feast of the Lamb (Revelation 19:7). And christ himself says he comes like a thief in the night when we least suspect it, thus we should always be prepared, as mentioned in Revelation 16:15:
So with the parable of the Bridesmaids and the Bridegroom, it makes logical sense being in a State of Grace or being in a State of Sin, and with being ready or not being ready. If you replace "State of Grace" with a doctrine like "By Faith Alone" it doesn't make any logical sense because then people are always ready. But I hope you can see from the above examples: the Spirit of God dwelling within us, the jewel hidden in the field, doing the will of the Father (producing good fruit), innocence of a child, and the parable of the Bridesmaids left behind; they all make perfect logical sense within the context of being in a State of Grace and what the Catholic Church teaches, and this is what the Catholic Church has taught for the past 2000 years.
So to wrap this up, if we look at Luke 17:20-21:
We can see Christ says that the Kingdom of Heaven is here with us on earth. Some translations say "is within you" while others say "in your midst." Regardless the net result is the same; the Kingdom of Heaven is right here with us, inside of us, in our midst, right here on Earth!