Is converting to another religion a bad thing?
The Bible is clear that there is only one way to reach Heaven: by faith in the Lord Jesus Christ. Jesus declared that He was the only way to the Father that no one comes to the Father except by faith in Jesus.
Furthermore, the Bible says that God’s standard for entry into Heaven is perfection: we must be as perfect as God to enter Heaven. Since all people have sinned and fall short of this standard (Romans 3:23), we must obtain the necessary perfection in another way. In His mercy God made a way for sinners to be credited with perfection by sending His Son, Jesus, to live a perfect life on earth as a man. When Jesus died on the cross, He was dying to pay for the sins of all who believe in Him, and by our faith in Jesus we may receive credit for His perfect life in place of our sins. This is the one true Gospel by which we may be saved and enter Heaven.
Therefore, should someone “convert” to faith in Jesus Christ, then it is a very good thing, for it means the person has been born again into the family of God and will be saved, as Jesus said:
John 6:40 “For this is the will of My Father, that everyone who beholds the Son and believes in Him will have eternal life, and I Myself will raise him up on the last day.”
On the other hand, if someone rejects this truth and decides to follow some other man-made, religious system (i.e., Buddhism, Catholicism, Mormonism, Islam, etc.), then it would be a very bad thing, since that person would have fallen for one of Satan’s many lies and having been persuaded to place their trust in something that cannot erase their sin or grant them the perfection required to enter Heaven.
Finally, if someone converts from one false religion to another (e.g., Buddhism to Islam, or Catholicism to Mormonism, etc.), then the result is meaningless. Trading one lie for another does not change the person’s eternal destiny. Only by coming to faith in Jesus Christ may a person be reconciled to God and receive assurance they will be welcomed into Heaven and receive salvation from judgment.