The writer of Hebrews warns us to be careful and make sure we don't fall away from Christ. This doesn't mean we need to strive in our own efforts to make sure we remain close to God. That will produce the opposite result. We strive; we fail; we get tired; and we end up far from God. We actually end up trying to operate under the old covenant law, rather than trusting in the new covenant grace of Jesus. If you want to remain close to Christ, don't depend on yourself, but trust that His grace is sufficient. He is faithful to complete the work He has started in you.