Let’s carefully consider the claim that king David’s sin with Bathsheba is proof believers can willfully commit terrible sins, still be forgiven and have confidence of Heaven. It’s common for sinners to comfort themselves with the idea that King David evidently sinned worse than they are, committing adultery and murder, yet he still made it. Is this reasonable?
It certainly is clear David committed terrible sins, he committed them willfully and persisted in them over a prolonged period, and he was also forgiven: he will be present in Heaven. But this does not prove the proposed claim; we don’t know for sure if David was a believer at the time he committed these horrible sins: it is conceivable he was regenerated afterward.
David did remarkable things in faith before his sin with Bathsheba, but this not necessarily an indication he was justified, regenerated by God. Some of Abraham’s great acts of faith were prior to his justification; he followed God’s call to leave his home country (Ge 12:4) before believing in God for salvation. (15:6) God doesn’t tell us exactly when David believed in God, having faith unto righteousness. It might have been after his sin with Bathsheba.
The Apostle Paul describes David’s understanding of salvation in no uncertain terms (Ro 4:6-8, Ps 32:1-2), yet this is likely (Spurgeon, vs 5) taken from David’s testimony after his sin with Bathsheba. Perhaps David’s sin is an indication he was not yet regenerated at the time.
Anyone living a life of willful sin who presumes they have eternal life from David’s example is banking on David committing his horrendous sins as a believer, but this presumption: it cannot be proven. What can be proven is what God tells us about believers and what they’re like. For one, no murderer has eternal life abiding in him (1Jn 3:15); one who premeditates a murder and carries it out, as David evidently did, gives clear evidence of an unregenerate state.
The Apostle John tells us to be very careful here: “Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him. Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous. He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil. Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God. In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.” (1Jn 3:6-10) John warns that false teachers will try to convince us we can live in sin and have eternal life; the Apostle Paul is also very clear here. (Ep 5:5-6, 1Co 6:9-10) It’s deception.
If we want to believe a lie, if we don’t love the truth, it will be nearly impossible to resist this one (2Th 2:11-12); it certainly appeals to the flesh. Let’s not fall for it, and gently warn those who are. (2Ti 2:24-26)