‘And I saw another angel ascend from the sunrising having the seal of the living God, and he cried with a great voice to the angels, to whom it was given to hurt the earth and the sea, saying “Do not hurt the earth or the sea or the trees until we have sealed the servants of our God on their foreheads”.'
Nothing on earth can hurt the people of God without God's permission, for all that would do so is restrained in one way or another by His power (compare 2 Thessalonians 2:6-7). The seal is intended to be recognised by angels and is thus invisible (Revelation 9:4). (It contrasts with the mark of the Beast in chapter Revelation 13:16).
‘The sunrising'. The idea may be that this takes place right at the beginning of events. As the sun begins to rise so the servants of God are sealed, before the activities of ‘the day' begin. Alternately it may signify the distant East (compare Revelation 16:12).
‘The trees'. The trees were important as providing sustenance to mankind. The third horseman was told not to ‘hurt the oil and the wine' which came from trees. Even invading armies were wary about destroying trees. Now it seems that that restraint is to be removed. This interlude is clearly preparatory for the contents of the seventh seal, where the earth, the trees, and the sea come under attack, but only to an extent limited by God (Revelation 8:7-8).
‘Until we have sealed the servants of God on their foreheads'. The picture is taken from Ezekiel 9. There great slaughter is to come on Jerusalem but it is restrained until a mark is set on the foreheads ‘of the men who sigh and cry of all the abominations that are performed in its midst', by the man clothed in linen holding the inkhorn (Revelation 9:3-4). They would be saved from the slaughter. The mark was not visible to men, only to God and the angels.
It can be compared with the mark in blood put on the houses of the Israelites to protect them from the angel of death (which, however, was visible) (Exodus 12:13). It is the mark of protection. It is not necessarily the same as the writing of the name in Revelation 3:12; Revelation 14:1, for that is to those who have already overcome and is a sign that they are Christ's for ever, although it may be that here it is used but not revealed. See Revelation 3:17.
Notice that it is ‘the seal of the living God '. It is not just a seal. It is the seal of One Who, as the ‘living' God, watches over His own. They belong to Him. It reminds us that nothing can touch God's people without His permission (compare ‘the hairs of your head are all numbered' (Matthew 10:30; Luke 12:7)). It is in strict contrast with those who will bear the mark of the Beast (Revelation 13:16-17).
The implication from the description ‘the servants of God' is that it is all God's servants who are on earth who will be sealed. The sealing saves them from the direct effects of attacks by spiritual forces, not from persecution (Revelation 9:4). So while they will suffer tribulation, they will not be subject to the wrath of God. What is important is that the men themselves know they are sealed by God. It is not suggested that there will be a visible sign.