15 And [kai] another [allos] angel [aggelos] came [exerchomai] out of [ek] the temple, [naos] crying [krazo] with [en] a loud [megas] voice [phone] to him that sat [kathemai]
on [epi] the cloud, [nephele] Thrust in [pempo] thy [sou] sickle, [drepanon] and [kai] reap: [therizo] for [hoti] the time [hora] is come [erchomai] for thee [soi] to reap; [therizo] for [hoti] the harvest [therismos] of the earth [ge] is ripe. [xeraino] KJV-Interlinear
another angel came out of the temple, crying out with a loud voice to Him who
sat on the cloud, "Put in your sickle and reap, because the hour to reap
has come, because the harvest of the earth is ripe." NASB
The key word here is ripe, 'xeraino,' which means to desecrate, to shrivel, to mature, to dry up, to pine away, to wither, to lose if not acquired now.
Believers were now ready, in undefined numbers, just as grain is ready for harvest. If the harvest is not done at the right time, then seed is lost, or it deteriorates and is lost.
Believers were ready for harvest. To do nothing, would only serve to throw away a good harvest. Likewise, the world was doing all it could to destroy them anyway.
God does not arbitrarily sweep people out of the world. People die from natural causes, from age, from accidents, from violence, and so forth.
In this case, believers were going to die as a result of unbelievable violence and hardships (starving, etc.) placed on them. And Christ will be there to collect them all, losing not a single individual, bring all safely to heaven.
The orders to harvest came from an angel, who came from heaven. In the logical hierarchy of heaven, the Father is the highest post, followed by the Son, followed by the Spirit. This chain of command is only for our benefit in order to understand the principles of authority and so forth. In reality all three members of the Godhead, are co-eternal, co-infinite, and co-equal.
However, the Father authored the divine plan, and the Son executes that plan.
Therefore, the Father issued the orders, via the divine plan, the angel (a messenger not identified) brought the orders to the Son, who executed the orders - harvest the souls, or better stated, collect the souls whose lives are being destroyed on earth by Satan, et al.
Remember too, that Christianity calls for access to the Father, through the Son. A very important principle which separates Christianity from all other religions and beliefs, who seek to bypass Christ, in arrogance, and go straight to the Father, or to some other deity concept (one with nature sort of stuff).
So, this reinforces the principle of the Father, Son relationship.
Therefore, Jesus Christ, in those last forty-two months of the Tribulation, will be busy harvesting a continuous flow of souls.