... And in his hand a sharp sickle. And another angel came out of the temple, crying with a loud voice to him that sat on the cloud, Thrust in thy sickle, and reap: for the time is come for thee to reap: for the harvest of the earth is ripe. And he that sat on the cloud thrust in his sickle on the earth; and the earth was reaped.
	Revelation 14:14-16
According to this passage, it would seem that there comes a point in the Tribulation when salvation is no longer possible. The 144,000 have been called to Heaven; the angels have made their proclamation; and now there will be a separation between those who become Christians in the Tribulation and those who don’t. So too, in your own tribulation you need to understand there comes a time when your own heart — even as a believer — can become hardened. The Bible speaks of a root of bitterness which can take hold in the soil of a man’s soul (Hebrews 12:15). I’ve seen believers go through tribulation and, rather than allow the Lord’s work of grace to take place in their hearts, choose instead to be bitter and unforgiving. Don’t let that happen. Don’t be unforgiving. Don’t be cynical. Don’t be bitter. Don’t play that game because if you let bitterness and unforgiveness continue, there will come a point in your own tribulation when it will become an irreversible part of who you are.