"Why do all of DJT’s biographers opine that he hasn’t any religious beliefs or principles at all? Why would evangelicals still support him if they are aware of this fact?"

 

Trump is not a Christian* based upon the definitive source of that word, (Acts 11:26) however, in a culture war both sides are looking for a general who will go to battle against the opposition, with such commitment overcoming personal integrity issues (which one side acts as if only the other one had).

And politically (versus hiring pastors of their church), in a culture war gone hot, then faced with a choice btwn two electable opposing candidates and lacking a saintly bulldog, evangelicals (who much supported Herman Cain till his fall, and at about 80% voted for a Mormon who has a  false god over Obama, and would even support a conservative atheist who strongly supports conservatives policies over liberals.

Which are essentially atheists anyway as regards the God of the Bible (note that atheists have one less god than Christians), as instead they are foremost proxy servants of the devil. But may they come to repentant faith in the risen Lord Jesus.

Hope this helps. Peace By Jesus

*The redeemed are those who come to God as sinners knowing their desperate need of salvation - not as souls saved by their works or religious heritage, but as destitute of any means or merit whereby they may find salvation (Romans 3:9 - 5:1) - and with a humble and penitent heart (that wants a new life following Christ) believe on the crucified and risen Lord Jesus who alone can save them on His account, by His sinless shed blood and righteousness.

And who are thus baptized to follow the Lord Jesus with persevering faith. And who thus are baptized and follow Him (John 10:27-29) 1 John 1:9; Hosea 5:15; 51:3; 6, Psalms 32:5; 2:8-10) - and effectually repent when they find that they failed to do so. (Ephesians 1:7; Acts 8:12;

For the redeemed are those are of penitent, heart-purifying, regenerating effectual faith, (Acts 10:43-47)Phil. 3:21; 2:6; cf. Ephesians 1:6) and by which faith the believer is completely forgiven and "accepted in the Beloved" and positionally seated with Him in Heaven. 28, Jn. 10:27; Acts 2:38-47) and is shown in baptism and following the Lord, (Romans 4:5; 15:7-9) which is imputed for righteousness, (

And wherever Scripture clearly speak of the next conscious reality for those who die in that effectual faith then it is with the Lord, at death or His return (whatever comes first), by the grace of God. (Lk. 23:43)1Thess. 4:17ff'; 1Cor. 15:51 [“we”]; 2Cor. 5:8; Phil 1:23]; Rv. 2:7; 2Cor. 12:4)