What does it really mean to believe with all your heart?
It means far more than a superficial mental assent; it is an honest, whole-being trust. It is believing without doubt that Christ died for you and rose from the dead to give you resurrection power. Scripture promises salvation to those who so believe, and by that faith alone one enters the relationship Paul expounds.
Les explains that 'only believe' does not mean a superficial mental assent. He says it means that when you honestly, with all your being, say, 'I believe this with all my heart, without a doubt.' He believes the Bible from cover to cover, but especially that when Christ died, He died for him, and when He rose, He rose to give him that same resurrection power. That is faith, and by that alone we enter the relationship Paul is always expounding. In the surrounding context, he also stresses that you do not win the lost merely by screaming or preaching at them; you win the lost by discipling the believer, so that a believer reflecting the righteousness of Christ prompts lost people to ask what they must do, giving the opportunity to tell them to believe Christ died for them, believe with all their heart He rose from the dead, and be saved.