This is the truth because it was spoken by the Lord Jesus: ‘Believe on the Lord Jesus Christ, and you will be saved.’It is not "different groups within Christianity" claiming it. It is a direct quote from God's word.
Acts 16:31 contains words spoken by the apostles to somebody enquiring about how to be saved. They replied simply, not in some mysterious dark sayings. What else could "Believe on the Lord Jesus Christ and you will be saved" mean, if it doesn't meant that belief on the Lord Jesus Christ is required for salvation? That does come from God. It is the same as what Jesus preached in John 3.
For your understanding of these words comes only from yourself, or perhaps from the understanding of other people; therefore, it cannot be called the truth.
Only what comes from God is the truth.”
JOHN 3:16
16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
Because the Jesus you know comes only from human explanations, you still do not truly know the Jesus who came in the flesh, as stated in 1 John 4:2–3.
1 OHN 4:2-3
2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
How can you believe in the true Jesus if you do not even know Him?