The Christian Ideal
The Christian ideal is a single marriage forever.
Condescension for the Laity
If a wife or husband dies, then although a new marriage is not encouraged (for Christians believe in the life after death and the general resurrection of the dead), it is allowed as a measure of condescension, voiced by the apostle, so that a person would not fall into some grave sins and then perish altogether:
“But I say to the unmarried and to the widows: It is good for them if they remain even as I am; but if they cannot exercise self-control, let them marry. For it is better to marry than to burn with passion” (1 Cor. 7:8-9)
“Therefore I desire that the younger widows marry, bear children, manage the house, give no opportunity to the adversary to speak reproachfully” (1 Tim. 5:14)
The Savior too, speaking of the prohibition of divorce, makes an exception for the case of fornication (adultery):
“Furthermore it has been said, ‘Whoever divorces his wife, let him give her a certificate of divorce.’ But I say to you that whoever divorces his wife for any reason except sexual immorality causes her to commit adultery; and whoever marries a woman who is divorced commits adultery” (Matt. 5:31-32) *
Thus, if a spouse has died or been unfaithful, a new marriage is possible. But still not endlessly, only as a measure of condescension: only two additional times.
To these reasons, by similar reasoning, the Church added some other reasons as well, such as a threat to life from the spouse and the like (in Russian).
A Man in a Single Marriage Must Be Chosen for the Priesthood
This is worth mentioning in order to see that remaining in one marriage is indeed considered a more worthy way of life:
“A bishop then must be blameless, the husband of one wife, temperate, sober-minded, of good behavior, hospitable, able to teach” (1 Tim. 3:2)
“Let deacons be the husbands of one wife, ruling their children and their own houses well” (1 Tim. 3:12)
“For this reason I left you in Crete, that you should set in order the things that are lacking, and appoint elders (Greek: πρεσβυτέρους — “presbyters”) in every city as I commanded you — if a man is blameless, the husband of one wife, having faithful children not accused of dissipation or insubordination” (Titus. 1:5-6)