There's a strand in American Christianity that's often called "me and Jesus" Christianity. It's usually found in Protestantism, but you'll sometimes see Catholics with a similar mentality. If you've ever heard anyone say that they love Jesus but hate the Church or that they believe Christianity is a relationship rather than a religion, there's a good chance you've encountered this kind of Christianity.
Essentially, it's the belief that as long as we have a relationship with Jesus, we don't need anyone or anything else in our spiritual lives. As the common name says, it's all about "me and Jesus." On this view, you don't need the sacraments, the priesthood, the saints, or anything else. Simply put, in this kind of Christianity, you don't need the Church.
The Gospel of Jesus
However, this is actually a distortion of Jesus' message. He didn't preach an individualistic religion focused entirely on our own personal relationships with God to the exclusion of anything and everything else. Rather, his message was all about community. Specifically, it was about the community that we today call the Church, so if we exclude the Church from our spiritual lives, we're actually rejecting the very Gospel that Jesus preached.
To see what I mean, let's take a look at how the Gospels summarize Jesus’ preaching. Matthew repeatedly says that he preached the "gospel of the kingdom" (Matthew 4:23, 9:35, 24:14), and Mark summarizes his message this way:
Jesus came into Galilee, preaching the gospel of God, and saying, "The time is fulfilled, and the kingdom of God is at hand; repent, and believe in the gospel." (Mark 1:14-15)
Similarly, Luke often tells us that Jesus preached "the good news of the kingdom of God" (Luke 4:43, 8:1, 16:16), and the Greek verb he uses for "preach the good news" literally means "preach the Gospel." As a result, it's clear that Jesus’ Gospel was about the kingdom of God.
The New Davidic Kingdom
Now, the Gospels never explicitly tell us what exactly the kingdom of God is, but if we look at the entirety of Scripture, both the Old and New Testaments, we find that it's the restoration of the kingdom of Israel. More specifically, it's the restoration of the kingdom ruled over by the descendants of King David, the man who defeated the giant Goliath and who later became the model Israelite king, and three lines of evidence point towards this conclusion.
First, back in the Old Testament, God promised David that his dynasty would last forever (2 Samuel 7:16), but it ended when Jerusalem was conquered by the Babylonians a few centuries later. As a result, the prophets foretold that God would one day raise up a new son of David to restore the kingdom and take the throne of his ancestors (for example, Jeremiah 33:14-15, 17, 20-21; Ezekiel 37:24-25; Hosea 3:4-5). This was Israel’s hope for a restored kingdom, so when Jesus came preaching the coming of a new kingdom, it stands to reason that this would be the kingdom he preached. Secondly, the New Testament explicitly tells us that Jesus is the fulfillment of this hope. It says that the promised son of David is Jesus (for example, Matthew 9:27, 15:22; Mark 10:47-48; Luke 1:31-33), so again, the natural corollary of this is that the kingdom he preached was the restored kingdom of David.
And finally, we can look at the Old Testament background to the phrase "kingdom of God." While we never see that exact Hebrew phrase in the Old Testament, we do see an equivalent one, "kingdom of the Lord." This phrase only appears twice, and both times it refers to the kingdom of David and his descendants (1 Chronicles 28:5, 2 Chronicles 13:8). From all this, it's clear that the kingdom Jesus preached was in fact the restoration of the kingdom of David from the Old Testament. All three lines of evidence that we've examined point to this conclusion, so it's pretty hard to escape. Jesus' message, the Gospel he preached, was all about this kingdom.
The Keys of the Kingdom
At this point, you may be wondering what any of this has to do with the Church. It may be great to know the focal point of Jesus' message, but it's tough to see how we get from the restored kingdom of David to the Church. However, bridging that gap is actually pretty simple. In short, the kingdom of God is the Church. Granted, the fullness of the kingdom will only come at the end of human history when Jesus comes again, but as the Catechism of the Catholic Church says, "The Church is the seed and beginning of this kingdom" (CCC 567). To see this, let’s look at two pieces of evidence. First, Jesus equates the Church with the kingdom in the Gospel of Matthew. He tells Peter:
And I tell you, you are Peter, and on this rock I will build my church, and the powers of death shall not prevail against it. I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven. (Matthew 16:18-19)
Jesus uses the phrase "kingdom of heaven" instead of "kingdom of God" in this passage, but they mean the same thing. In Matthew's Gospel, Jesus tends to call it the "kingdom of heaven" in order to emphasize that it comes to earth from heaven, but the two phrases refer to the same kingdom. A few chapters later in Matthew, he uses both of them synonymously (Matthew 19:23-24), so there's no doubt about this.
With that red herring out of the way, let's take a look at Jesus' equation of the Church with the kingdom. He first says that he's going to build his Church on Peter, and then he says that he's going to give Peter "the keys of the kingdom of heaven." Now, he doesn't explicitly say that the Church is the kingdom, but it's the obvious implication of what he does say. The most natural interpretation of Jesus' words is that Peter's two roles are related. He's the foundation of the Church precisely because he has the keys of the kingdom, which implies that the Church is in fact the kingdom.
The Davidic Church
For further confirmation of this, we can turn to the Acts of the Apostles. In the early Church, there was a controversy over whether Gentile (non-Jewish) converts to Christianity had to follow the Law of Moses, so a council was called in Jerusalem for the Apostles and the leaders of the Church to discuss the issue. They eventually decided that Gentile Christians didn't have to follow the Law, and one of their reasons was based on an Old Testament prophecy:
After this I will return,
and I will rebuild the dwelling of David, which has fallen;
I will rebuild its ruins,
and I will set it up,
that the rest of men may seek the Lord,
and all the Gentiles who are called by my name,
says the Lord, who has made these things known from of old. (Acts 15:16-18, quoting Amos 9:11-12)
This passage foretells the restoration of the kingdom of David, and for our purposes here, the important thing is that the Apostles used it to reach a conclusion about the Church. They were saying that since the restored kingdom would include Gentiles, the Church shouldn't force Gentiles to follow the Law of Moses (the unspoken assumption was that if Gentiles followed it, they would technically become Jewish, since it was the national law of the Jewish people). And if that's the case, then there can only be one conclusion: the Church is the restored kingdom of David, the same kingdom that Jesus preached. Otherwise, this text wouldn't have any bearing on the question the Apostles were discussing.
No Church, No Jesus
Now, if the Church is the kingdom of God, and if Jesus' Gospel was all about the kingdom of God, then Jesus' Gospel was actually about the Church. Granted, he didn't explicitly lay out a blueprint for everything that we think of today when we talk about the Church (like the sacraments, the hierarchy, church buildings, etc.), but he did talk about its essence, the body of followers that would constitute it. Consequently, if we reject the Church, we're actually rejecting the message of Jesus; if the Church has no place in our spiritual lives, we're rejecting the very Gospel that he preached. Simply put, if we want to follow Jesus, we need the Church.