Question: "Should a Christian co-sign on a loan?"
Answer: In answer to the question of whether a Christian should co-sign a loan, the passages from the wisdom literature in the book of Proverbs come to mind. If one believes in the inspiration of the Scripture (that it is God-breathed), even Solomon's words of wisdom come from God. Proverbs 22:26-27 states, "Do not be a man who strikes hands in pledge or puts up security for debts; if you lack the means to pay, your very bed will be snatched from under you.” This would seem to indicate that God advises against such a practice.
Proverbs 11:15 states: "He who puts up security for another will surely suffer, but whoever refuses to strike hands in pledge is safe." These words were written centuries ago to warn men of what is still very common ground for failure in life and ruin in business. To put up security for a stranger is a most dangerous thing, as thousands have learned to their sorrow. Many a man will do this for a friend long known and trusted; but no wise man will act for a stranger. Often, parents will sign notes or loans with their children. There is a principle seen in these biblical words of wisdom. No one should co-sign (guarantee) with another on any amount which possibly could fall back upon them and become their responsibility to pay.
Whether or not it is wise or safe to co-sign a loan for a relative or trusted friend is difficult to say, and much depends on the circumstances. One needs to be discerning, commit the situation to prayer, and follow the Lord's leading where known and trusted people are involved, and one must be able to pay should it ever become necessary. But this is definitely not something to do for strangers and those barely known to us.