How do you know Mack is saved and only suspect that Dave is? What evidence do you have on anyone other than yourself? And then how can you know that your evidence is pleasing to God?
1 Corinthians
Chapter 4
1 Thus should one regard us: as servants of Christ and stewards of the mysteries of God. 2 Now it is of course required of stewards that they be found trustworthy. 3 It does not concern me in the least that I be judged by you or any human tribunal; I do not even pass judgment on myself; 4 I am not conscious of anything against me, but I do not thereby stand acquitted; the one who judges me is the Lord. 5 Therefore, do not make any judgment before the appointed time, until the Lord comes, for he will bring to light what is hidden in darkness and will manifest the motives of our hearts, and then everyone will receive praise from God.
Rev 12:11 And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death.