I'm a bit confused about what Jesus is saying about how one gets to heaven.
In John 3:16 he says that we just need to believe in God and we will be saved. And that there is no judgement for those who believe in Jesus. Yet, in Mathew 7:21 Jesus says that not everyone who calls out "Lord, Lord" will make it to heaven, but only the people who do the will of God. Even though it seems like these people he is talking about believed in God since they are claiming to have "cast out demons in Jesus name" etc. But if people need to "do God's will" to make it into heaven, that would be like saying that Jesus dying on the cross wasn't enough to save us but there is still stuff we must do to achieve salvation.
Also, in Mark 1:15 Jesus says the kingdom of God is near. Repent and believe the good news. Does that mean to make it into heaven we have to repent? In this case as well, that would mean we must do something on our part besides believe in Jesus to be saved.
And with all this in mind, it still says that those who believe will not be judged, so it seems a bit contradictory.
Sorry for the long question. I'm just trying to wrap my brain around this.
Thanks for your time.
Your question is touched on in the article: What Must I Do to be Saved? What I will do in this response is address the particular passages and issues that you raised.
Jesus died for everyone. "For the love of Christ compels us, because we judge thus: that if One died for all, then all died; and He died for all, that those who live should live no longer for themselves, but for Him who died for them and rose again" (II Corinthians 5:14-15). If all that it took was Jesus' death, then everyone will end up in heaven. But we know this is not true. "But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death" (Revelation 21:8). The flaw is rooted in an assumed "only" being used in regards to Jesus' death. It is true that no one could be saved without Jesus' death and resurrection, but the passages do not say that this is the only thing required.
Even you added a condition, but you didn't consider it deeply because you heard this all your life. "For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life" (John 3:16). There is a condition of believing. Belief is not something done to you. It is something you do; yet, it does not earn you salvation. It is something that God requires before He will give the salvation that was earned by Jesus' death. "Jesus answered and said to them, "This is the work of God, that you believe in Him whom He sent"" (John 6:29).
Faith is not the only work required by God. Many people can quote Ephesians 2:8-9, but have you noticed the very next verse? "For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast. For we are His workmanship, created in Christ Jesus for good works, which God prepared beforehand that we should walk in them" (Ephesians 2:8-10). This then explains Jesus statement that refusing to do the works of God bars a person from salvation. "Not everyone who says to Me, 'Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of My Father in heaven" (Matthew 7:21). To claim Jesus as "Lord" is to state that he is your ruler. "But why do you call Me 'Lord, Lord,' and do not do the things which I say?" (Luke 6:46). Words are not enough. Belief has to be so strong that it is seen in what a person does. "What does it profit, my brethren, if someone says he has faith but does not have works? Can faith save him? ... Thus also faith by itself, if it does not have works, is dead." (James 2:14,17).
If you examine all the passages dealing with salvation, you will find a number of things that God requires from man. See: What Saves a Person? Thus, yes, repentance is required. "I tell you, no; but unless you repent you will all likewise perish" (Luke 13:3). It is not a work that earns a man salvation. It is a work of God done to meet God's requirements.
The same is also true of baptism. "In Him you were also circumcised with the circumcision made without hands, by putting off the body of the sins of the flesh, by the circumcision of Christ, buried with Him in baptism, in which you also were raised with Him through faith in the working of God, who raised Him from the dead" (Colossians 2:11-12).
The passage in John 3 continues by saying, "For God did not send His Son into the world to condemn the world, but that the world through Him might be saved. He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God" (John 3:17-18). Jesus did not come into this world to bring it into judgment or condemnation. It already stood condemned. His purpose was to rescue people from that pending condemnation. But this does not mean judgment isn't coming. "For we must all appear before the judgment seat of Christ, that each one may receive the things done in the body, according to what he has done, whether good or bad" (II Corinthians 5:10).
A person who truly believes does as his Lord directs. "Jesus answered and said to him, "If anyone loves Me, he will keep My word; and My Father will love him, and We will come to him and make Our home with him"" (John 14:23). Since belief is demonstrated in what we do, our actions become the basis of our judgment. A true believer has no concern about condemnation because he is devoted to doing the works of God.