Catholics believe that Jesus Christ is The Way, The Truth, and The Life. No one comes to the Father except through Him. Not everyone will enter Heaven; the gate is narrow and the way is hard, that leads to life, and those who find it are few. Catholics have a sacred duty to spread the Gospel and tell everyone about Jesus Christ. No one who knowingly rejects Jesus Christ will enter heaven.
Anyone who enters Heaven, does so because of Jesus Christ but Heaven is not restricted to only those who have an explicit faith in Jesus Christ:
Infants and small children and some mentally retarded people are not capable of having an explicit faith. If they die before reaching the age of reason, are they to be condemned to Hell? Not necessarily. Jesus said that the kingdom of Heaven belonged to such as these.
The holy man and women in the Old Testament (the prophets, etc.) did not have explicit faith in Jesus Christ. Had they been given the opportunity they surely would have but they never had the opportunity. Were they condemned to Hell? No. (See Hebrews 11:1—12:24) They entered heaven after Jesus’ death on the Cross because during their lifetime they responded to the graces that God gave them and strove to do his will in everything as they understood it.
Today, man and women who through no fault of their own do not have an explicit faith in Jesus Christ but who nevertheless respond to the graces that God gives them and strive to do His will in everything as they understand it, like the holy men and women of the Old Testament, may also enter heaven.
Suggested reading: Catechism of the Catholic Church: “Outside the Church there is no salvation” (846-848) and Mission - requirement of the Church’s catholicity (849-856)