In I Corinthians 10:12 Paul states: "Wherefore let him that thinketh he standeth take heed lest he fall." Then in II Peter 1:10 "Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall." Can you explain the difference?
In both verses, there are qualifications or conditions to the statements that are essential for understanding.
In I Corinthians 10:1-4, Paul shows the Corinthians that they have no greater advantage over the Israelites under the Old Testament. It's easy to think that if you lived in those days you would never sin as they did. In I Corinthians 10:5-11, Paul points out the reasons why the Israelites who escaped the slavery of Egypt didn't make it into the promised land. Different people fell for different reasons. But do you realize that conservatively over 2 million people left Egypt and of that number only two of the original adults who left entered Canaan? Paul's point is that just because you have been saved, it isn't a guarantee that you will make it to heaven. Too many people become lax because they think they have it made and drop their guard. "Therefore let him who thinks he stands take heed lest he fall" (I Corinthians 10:12).
Peter is covering the same basic point. It is possible for a Christian to fall away. "For he who lacks these things is shortsighted, even to blindness, and has forgotten that he was cleansed from his old sins" (II Peter 1:9). But Peter doesn't want it to come across that falling away is inevitable. "Therefore, brethren, be even more diligent to make your call and election sure, for if you do these things you will never stumble; for so an entrance will be supplied to you abundantly into the everlasting kingdom of our Lord and Savior Jesus Christ" (II Peter 1:10-11). Because it is possible for you to lose what you have gained, it ought to spur you to greater action to make sure you hang on to what you have. How? "If you do these things you will never stumble." If you constantly grow and apply the things Peter discussed earlier (II Peter 1:5-8), then you won't stumble, you won't fall away. People who fall away stop following the Lord's commands. They stop growing as Christians. Therefore, keep working at it and you won't fall. "For this reason I will not be negligent to remind you always of these things, though you know and are established in the present truth" (II Peter 1:12).
Therefore Peter and Paul are stating the same thing, but coming at the same issue from different viewpoints.