What do we mean by 'meaning' when we ask about the meaning of life? It is perhaps most natural to take the meaning of life or of a life to be its purpose, point, end, goal, or telos. Accordingly, (human) life is meaningful only if it has a central organizing purpose. Existential of life meaning bears a teleological aspect in that a meaningful life is a purpose-driven life.
Having a purpose, even if necessary for the meaningfulness of a life, is not sufficient. A meaningful life must also embody positive intrinsic value. The lives of terrorists and mass murderers can be purpose-driven, subjectively meaningful, and satisfying to their agents, but we ought to resist the notion that such lives are objectively meaningful. At best, such destructive lives are subjectively meaningful only. If so, existential meaning is not merely a teleological concept but a teleological-cum-axiological concept. An objectively meaning ful life must be both purpose-driven and such as to realize positive objective intrinsic value.
Even if one's overall purpose is not destructive, a life devoted to a dominating purpose might nonetheless lack positive objective intrinsic value if the purpose is either trivial or futile. One hesitates to say that a life devoted to parsing every sentence in Moby Dick is objectively meaningful: such a life is squandered on a trivial pursuit. Intuitively, a purpose-driven life can be a wasted life. Or suppose the cynosure of one's efforts is something unattainable such as the squaring of the circle or the construction of a perpetuum mobile. A life devoted to the unattainable might not be trivial but it will be futile.
The futility/triviality contrast can be thrown into relief by comparing religious and nonreligious lives. Thomas Merton joined the Trappists as a relatively young man, and Edith Stein, the brilliant student of Husserl, entered the Carmelite Order. Did these outstanding individuals throw their lives away? Both lives were consecrated to the highest conceivable purpose: to serve, love, and know God. Their common purpose was the exact opposite of trivial. But the worldling will say that their lives were exercises in futility: they aimed at the unattainable since there is no God. There is only this world, and all attainable purposes are immanent to it. Merton and Stein walled themselves off from the only life there is and spent their lives in illusion. They renounced the very things that make life worth living: family life, children, freedom, career, sex and the pleasures of the senses.
But if the worldling is wrong, then no matter what world-immanent purpose he pursues, no matter how highly esteemed the purpose is in the eyes of the world, his life will be trivial. For if God exists, then any goal short of God is utterly trivial in comparison. ("What doth it profit a man, etc.") So if God exists, the worlding wastes his life just as surely as the committed religionist does if God does not exist.
There are at least four ways a life can be meaningless. A life is rendered meaningless by purposelessness or directionlessness. But a life is is also rendered meaningless by being misdirected in one of three ways: either misdirected to an evil purpose, or misdirected to a trivial purpose or misdirected to a futile purpose.
So I think we can safely reject this biconditional: A human life is meaningful if and only if its constituent activities, events and projects are animated and organized by a central purpose. To get a truth we must replace 'if and only if' with 'only if': having a purpose is necessary but not sufficient for existential meaningfulness. This implies that existential meaningfulness cannot be defined solely in terms of teleology. Axiology must be brought in.
But I hear an objection coming. "What you are saying is right if by 'meaningful' you mean objectively meaningful. However, it seems correct to say that a life is subjectively meaningful if and only if it is organized by a central purpose, one supplied by the agent of the life."
I reject the notion of subjective meaning. To say that a life has a subjective meaning is to say that it has no meaning at all. For if I must give my life meaning, then it has no meaning.