Question: "Should a Christian co-sign on a loan?"
Answer: I'm immediately reminded of the passages from the wisdom literature in the book of Proverbs. If one believes in the inspiration (God-breathed) of the Scripture, even Solomon's words of wisdom come from God. Proverbs 22:26, 27 state, "Be thou not one of them that strike hands, or of them that are sureties for debts. If thou has nothing to pay, why should he take away thy bed from under thee?" Proverbs 6:1, 2 says, "My son, if thou be surety for thy friend, if thou hast stricken thy hand with a stranger, thou art snared with the sayings of thy mouth, thou art taken with the sayings of thy mouth" (See verses 3-5).
Proverbs 11:15 states: "He that is surety for a stranger shall smart for it: And he that hateth suretyship is sure." These words were written centuries before the Cross to warn men of what is still a very common ground for failure and ruin in business life. To surety for a stranger is a most dangerous thing, as thousands have learned to their sorrow. A surety is one who goes good for another. 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 cosign (guarantee) with another on any amount which possibly could fall back upon their responsibility to pay.
I think, however, there was One who knew to the full what all the consequences of His act would be, and yet, in grace, deigned to become "Surety for a stranger." "Ye know the grace of our Lord Jesus Christ, that, though He was rich, yet for your sakes He became poor, that ye through His poverty might be rich" (2 Corinthians 8:9). He was the stranger's Surety. It was when we were "strangers and foreigners," "enemies and alienated in our minds by wicked works," that Jesus in grace became our Surety. He "died, the Just for the unjust, that He might bring us to God," All we owed was exacted from Him when He suffered upon the tree for sins not His own. He could then say, "I restored that which I took not away" (Psalms 69:4). I can't say that it is or isn't. If you follow after wisdom, it would not be biblical in the context of the passages above. You have to decide whether or not it would be a wise move or a safe one. One needs to be discerning and follow the Lord's leading where known and trusted people are involved, and be able to pay should it ever become necessary. Definitely not for strangers and those unknown to you.