Sometimes strength is not in what we think it is, especially spiritual strength. We look at the preacher and his family and assume they must be strong because he is the preacher. We look at elders and think surely they are strong or they would never have become elders. We look at those who teach in a "preacher school" and know they are strong because they are teaching others about the Bible. We see someone who "never misses church" and figure they are strong. The problem is that while those things might be evidence toward believing someone is strong they are not the source of spiritual strength.
God and His Word are the only true source for spiritual strength. We may be encouraged by others, but we can never use someone else's strength when we are out there facing temptation alone. There were some commercials a few years back that showed someone who seemed to be in perfect health, but then they would collapse because of dangerous levels of cholesterol. The point of the commercials was that you can't always tell if someone is healthy by looking at the outside. The same is true of a persons spiritual life. Jesus chided, no, blasted the Scribes and Pharisees because they appeared to be pious and righteous to men by the things they showed, but inside they were rotten to the core (Matthew 23).
In recent years there have been a number of athletes who were young and seemingly in prime condition who have dropped dead because of heart problems they didn't know they had. We have to realize that our heart is an essential part of being right with God. We can't do the right things for the wrong reasons and have spiritual strength. We may look strong and healthy to others, but sooner or later we will drop dead spiritually from a heart attack.