How do we know that the gospel is free and without ulterior motives?

In his letters to the Corinthians, Paul makes it clear that his preaching of the gospel was done freely and without any ulterior motives, as he took no financial support from them but relied on other churches instead (2 Corinthians 11:8-9). This approach was deliberate to set an example against the false preachers who were known for their greed and manipulation of the church for financial gain. By preaching freely, Paul emphasizes that the message of salvation is a gift from God, meant for all to receive without cost or manipulation, reinforcing the biblical principle of grace alone in the context of salvation.