And I saw HEAVEN OPENED;
This may seem strange, but apparently heaven is not a wide-open place. Rather, at least with respect to this sin-riddled earth, heaven is protected from evil (Matthew 3:13-16-17, Acts 7:54-56-60, Revelation 4:1, and Revelation 15:5). A person should be aware that Hades is within sight/hearing distance of Abraham's boson (heaven?). The need to protect heaven from evil should also make us much more alert to the foibles of the devil The devil is very crafty and will be able to deceive many (like the sand of the seashore - Revelation 20:8) after the millennial reign of Christ.
This may also explain why the New Jerusalem (heaven) has gates (Revelation 21:12).
and BEHOLD, a WHITE HORSE, and HE who sat upon it
The WHITE HORSE symbolizes power and warfare from a righteous, pure, and holy army. The commander-in-chief, here denoted as HE, is none other than Jesus Christ. While we have seen how Jesus Christ looks in heaven (Revelation 1:12-16), here we find Him in his "military" role and as Messiah (John 4:25-26).
In human terms, we never can be FAITHFUL and TRUE because of the sin curse placed upon us. So we hear phrases like "fair" or "impartial" used; but even these are hard to confirm. Comparing among earthlings always leaves room for untruth. Not so with Jesus Christ; He is CALLED (by God, we assume) FAITHFUL and TRUE;
Because Jesus is TRUE, He is able to JUDGE and execute this WAR against evil. We, as His supporting army, should have been faithful during our basic training (Ephesians 6:10-12-20). Then we will be able to wage war with Jesus. Sadly, too many so-called Christians never go to 'boot camp'; it is doubtful they'll be part of the army (verse 14).