If Jesus is referring to baptism in John 3:5, then it is undeniable that nobody will go Heaven without water baptism (being born of water). However, the Catholic Church maintains that it is entirely possible for one to go to Heaven without water baptism. How is one to resolve this tension?
Jesus does not use the term “heaven”, he uses “kingdom of God”.
"…the phrase
kingdom of God can be used in more than one sense.
In the most general sense, the kingdom of God would seem to be everything under God’s sovereignty, which is all of reality. However, you can’t get away from the kingdom of God in this sense, so most folks mean something more particular when they use the phrase.
In a more restricted sense, one might say that the kingdom of God comprises those things in which God is specially active—i.e., is asserting his sovereignty. In this case, since it requires a miracle every time the Eucharist is celebrated, one would say that the Mass involves the kingdom of God in a special way.
Scripture speaks of the kingdom of God as if it is present in the Person of Christ, and since Christ is present in the Eucharist, the kingdom of God is present in the Mass in that way.
Scripture also speaks of the kingdom of God as if it is an age of history that is inaugurated with Christ. We are living in that age now, the Mass being one of its characteristic features.
Scripture also speaks of the kingdom of God as if it is a future age that begins with the Second Coming of Christ. In this case, since the Eucharist involves a making-present of Christ in anticipation of the Second Coming, it would seem that there is a sense in which the future kingdom of God is made present in the Eucharist."
catholic.com/thisrock/2002/0207qq.asp
As for the salvation of the unbaptized, God’s justice is above private interpretation of a few bible verses. Only a mean angry Nordic sky god would create people just to send them to hell. “For God so loved the world…”, (John 3:16) not just Christians.
Remember that Jesus said from the cross, “Father, forgive them; for they know not what they do” (Lk 23:34). What did his sacrifice on the cross have to do with those who mocked him, spit at him, and crucified him? In other words, what did it have to do with those for whom he was asking forgiveness from God the Father? The answer is: everything. For some, it was through ignorance that they crucified him, and invincible ignorance can diminish one’s culpability for evil done—even the crucifixion of our Savior. They still could be saved, or Jesus’ plea for their forgiveness to the Father would make no sense. Whoever enters heaven will do so only because of Christ and his sacrificial work on the cross. Some people may enter heaven having never heard of Christ, but it will be because of him that they enter.
catholic.com/thisrock/quickquestions/keyword/Redemption
There is no tension.