I am Pentecostal and I remember hearing someone say that most Catholics will probably go to hell because they don't ask Jesus into their hearts for forgiveness. Is this true?
Sinners, whether they claim the designation Catholic, Pentecostal, church of Christ, or anything else, will receive eternal punishment if they are not forgiven of their sins. Those who are forgiven by God's grace will be with him forever. That is the long and short of it.
The problem comes in determining how God chooses whom to forgive. For that we have to look at what He says in His word, the Bible. In that respect it is significant to note that the Bible never once speaks of "asking Jesus into your heart." Not only is this a non-Biblical phrase, it is very misleading about what God requires for salvation.
First let me point out that what follows is NOT saying that we can do anything to earn salvation or forgiveness of sins. That is a gift of God that we would not deserve, no matter how much we do. "But we are all as unclean, and all our righteousnesses as filthy rags." (Isaiah 64:6) On the other hand, God has established certain requirements in order for us to receive that salvation. It's like a Christmas present under the tree. It has your name on it; it is a freely offered gift, and nothing you do can earn it. However, until you take it out from under the tree and open it, it may as well not have been given to you. God requires that you open his gift.
Most people who talk of "asking Jesus into their hearts" will admit that one of God's requirements is faith. "But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him." (Heb 11:6) Believing in God is one of the "works" (as some try to state it) that God requires. But it is not the only one. "Thou believest that there is one God; thou doest well: the devils also believe, and tremble." (James 2:19)
Those who would "ask Jesus into their hearts" usually do so through prayer, asking God to forgive them. Prayer is good, but God never says that he will forgive anybody just because they pray to him. Jesus even said it wouldn't happen that way: "Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven." (Matt 7:21) He says that beyond prayer there is something to "do."
Most will accept that after we believe we must also publicly acknowledge that belief. "Whosoever therefore shall confess me before men, him will I confess also before my Father which is in heaven. But whosoever shall deny me before men, him will I also deny before my Father which is in heaven." (Matt 10:32-33)
Where most people fall short, and Catholics on the other hand agree with the scriptures, is the necessity of immersion in water (baptism) for the forgiveness of sins. In the first recorded gospel sermon Peter told his hearers that in order to be saved they must "repent, every one of you, and be immersed for the purpose of forgiveness of sins." (Acts 2:38) Those who say you just have to "ask Jesus into your hearts" should look at Acts 22:16. "And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord." Ananias told Paul (at the time called Saul) that immersion served two purposes toward salvation. Firstly, it is in that immersion that our sins are washed away. Without that washing we are still in sin, and therefore liable to eternal punishment. Secondly, it is that immersion and not prayer that is the act of "calling on the name of the Lord." Paul had been praying for three days, but sincere prayer did not effect the forgiveness of his sins. So in the biblical sense, Catholics are more likely to have "asked Jesus into their hearts" than many non-Catholics because they are more likely to have submitted to baptism for the forgiveness of sins (although many have not, because they were not immersed).