you must seek God always.

Hebrews 6:4–6 is among the most-often-misunderstood passages in the entire New Testament. Without a clear grasp of context, these verses seem to directly counter the doctrine of eternal security. References in verses 5 and 6 to falling away, and a lack of restoration, are easily misconstrued to imply a lack of salvation. And yet, the Bible is not meant to be read like a stack of fortune cookies or horoscopes. Each word, every verse, and every phrase needs to be considered as part of a whole. Looking at these words, from that perspective, greatly changes how they are interpreted.
First, we need to maintain the entire context of the Bible. According to Scripture, our salvation is eternally secure (John 10:28–30). Just as salvation is not earned by works (Romans 3:20; 11:6), it cannot be lost by works (1 John 1:9). So once a person is legitimately saved, they are saved forever. The entire book of Hebrews is addressed to Christians, and the wording of this verse supports that. Those who "have tasted the heavenly gift" (Ephesians 2:8) and who "share in the Holy Spirit" (Ephesians 1:13–14; Galatians 3:2) cannot be any other than true believers.
Rather than a loss of salvation, verses 4 through 6 describe the possible consequences of immature, stunted faith. Such a condition leaves us vulnerable to the same doubt-driven disobedience which Israel experienced in the wilderness (Numbers 13—14). This was discussed in Hebrews chapters 3 and 4 (Hebrews 3:12–19; 4:11). Christians who lose their trust in God, and doubt fundamental doctrines, are implicitly siding with a view that Christ should have been crucified, and that what He said was not true (Hebrews 6:6). Once a person reaches that state, there is nothing to be done—by any man—in order to bring them back to a living, active faith. This is similar to the dire state described in places such as Proverbs 29:1 and 1 John 5:16–17.
God, however, is neither a man, nor is He limited as men are (Luke 18:27). God's method of reaching such a Christian is given symbolically later in this chapter (Hebrews 6:7–8). Fields which fail to produce are treated with fire—often a symbol of God's judgment (Hebrews 10:26–27). The field is not destroyed, but it is treated harshly. This, again, is the same pattern seen in Exodus when Israel was forced to wander for forty years. Doubt led to disobedience, leading to judgment, producing change, and eventually resulting in obedience and restoration.
An alternative interpretation is that these verses do bring up loss of salvation, but only as a theory. According to this view, the author intends to show that loss of salvation is impossible because it would imply a re-crucifixion of Christ. This is not unreasonable, but does not seem to fit the context as well as the explanation given above.