Why do Pentecostals teach that if don't speak in tongues you will go to hell?
Thank you so much for this great question. First of all let me tell you that the Bible does not require tongues as a prerequisite of heaven. But yu asked why some believe it is. I believe that the confusion comes from understanding what it means to be indwelt by the Holy Spirit. The Bible does tell us that if one does not have the Holy Spirit living inside them, then they are not saved.
“For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit. For to be carnally minded is death; but to be spiritually minded is life and peace. Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be. So then they that are in the flesh cannot please God. But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.” (Romans 8:5–9, KJV 1900)
According to this passage, the evidence of the indwelling Holy Spirit is not tongues, but a change of appetite from a carnal mind to a Spiritual mind. It is the presence of a new fight between the carnal fleshly appetites and the Spiritual ones.
“For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.” (Galatians 5:17, KJV 1900)
Only three times in the Bible do we have specific examples of people speaking in tongues. To require this as evidence of salvation goes beyond Biblical teaching.
I hope that I have helped you. I am so very sorry for my late response.