Dr. R.C. Sproul, commenting on today's passage in his commentary Mark, writes, "We all need to examine ourselves in the light of God's Word to be sure the deadly leaven of the Pharisees is not working in us, blinding us to the light and making us deaf to the life-giving Word." This speaks to the necessity of not taking our relationship to the Lord for granted, of not assuming that we are immune to the destructive effects of sin simply because we are in church regularly or have professed faith for many years. Merely being in the presence of Jesus will not make us resistant to sin if we do not understand and apply what He has revealed.
That principle is illustrated in Mark 8:16–21 as Jesus chides the disciples for getting caught up in their concern regarding their lack of bread. Such worry and prolonged discussion over the matter (see vv. 14, 16) is possible only because the disciples still lack the kind of faith that characterizes those who are fully committed to Jesus. After all, if they were to understand truly His identity, certainly they would just ask Him to meet their need for bread. They have seen Him feed thousands of people with only a meager amount of food on two occasions already, and that is what makes their shortsightedness so frustrating to our Lord (vv. 19–21; see 6:30–43; 8:1–10).
Given that the leaven of which Jesus has warned the disciples is the unbelief of the Pharisees and Herod (8:14–15), we see that unbelief is possible even for those who see Jesus with their own eyes. Knowing the works of Christ and what they say about His person is vital, but in itself, such knowledge is incapable of guaranteeing faith in Him. In fact, while such knowledge is necessary for true faith, this understanding is also dangerous because it gives the heart an opportunity to harden itself against divine truth. The scribes and Pharisees responded with hardness (3:22–30), and although those who truly know Christ in a saving way can never finally harden themselves in such a way that they lose their salvation (Rom. 8:29–30), it is possible for believers to harden themselves against some aspects of God's revelation, at least temporarily. The only answer is to continue returning to God's Word, hearing it preached and reading it ourselves. As we do so, we must pray that God would soften our hearts to see where we have hardened them. He gives us new hearts to believe, and we rely on Him to keep our hearts soft and receptive to the conviction of His Word.
Simply being in the presence of Jesus is not enough to guarantee faith. Instead, God must grant us ears to hear and hearts to believe His Word. We must not only attend to the preaching and reading of Scripture, but we must also pray that the Lord would open our eyes and minds as we do so. We need His work in our hearts so that we will believe and obey His Word.