In any task it is good to begin well. But it is at least equally important to finish well. Some of the saddest stories in the Bible have to do with those who began well but left their first love (Rev 2:4), ceased to be devoted to God (Jer 2), and ended in disaster.
I do not think that anyone sets out to be an apostate. Those who fall away do so slowly, more by neglect than by intention. Drift will always take us downstream. No one has ever drifted to the headwaters of a stream. No one ever reaches his maximum potential effortlessly or thoughtlessly. If we desire to remain firm in our faith there are certain things we must maintain.
Acts 2:42 sums up what will be needed in three simple points. The concepts are simple, but the accomplishment will require persistence.
To maintain our first love, we must remain "devoted to the apostles' teaching". Generally, those who drift from the Lord have first drifted away from the habit of daily Bible study. Maybe they think that they know enough already. Maybe they think that the proverb, "Cease to hear instruction, my son, and you will stray from the words of knowledge" (Prov 19:27), does not apply to them. It applies to all of us. Without constant contact with God's instruction, we will drift into worldly thinking and fall away.
To maintain our first love, we must remain "devoted to the fellowship." Apostasy does not begin when we commit some major sin. It begins when we no longer care to break bread with our fellow believers, when we find the fellowship of the saints something we can take or leave. Those who love the Lord will naturally love his body. Those who drift away from the assembly of the saints (Heb 10:25) are almost certain to drift away from the fundamental truths of the gospel (Heb 2:1).
To maintain our first love, we must remain "devoted to the prayers." The Bible is too powerful of a book to be handled without prayer. Fellowship, even with other believers, needs guidance. We must not depend on ourselves. We must not depend on our fellow Christians. We must depend on the Lord and express that dependence in prayer.