It is clear from the Parable of the Good Samaritan that Christian love for neighbor must extend beyond the confines of the Church. The First Letter of John says many beautiful things about how our love for one another is necessarily intertwined with our love for God: we love one another in the relevant way if and only if we love God, and this love is the center of the Christian life. But a puzzling feature of the Letter is that the love for one another appears specifically to be a love only within the Christian community: it is love for another Christian.
One way of reading 1 John together with the Parable is hierarchical: yes, we have a duty to love all our neighbors, including non-Christians. But the love for fellow Christians is the apex of this love, and it is only in the love for fellow Christians that the love for God can fully spread its wings, which is why the author of the Letter focuses on it. Furthermore, one might add something about the specific purposes of the Letter tied to disturbances within the community it was addressed to.
That all may be a part of the truth. But I want to propose something that goes a little further. We love our non-Christian neighbor as someone with a potentiality for being a fellow Christian. And not just a mere potentiality, such as my potentiality, which I expect will never be actualized, for tap-dancing outside of St. Stephen's Basilica in Budapest (certainly, I have the capabilities for learning some rudimentary tap-dancing and traveling there—but I don't expect to actually do it), and it is no great loss that it will not be actualized. Rather, it is more like the fetus's potentiality for becoming an adult, in the sense that it is a potentiality that is grave loss to the individual when it is not actualized, a potentiality that is not merely a matter of possibility, but a matter of an impulsion to the end. But there is a difference between the non-Christian's potentiality for becoming a member of the body of Christ, and the fetus's potentiality for becoming an adult. The fetus's potentiality for becoming an adult is a natural power in the fetus that simply needs the right environment. The non-Christian needs grace, which God offers to all.
That we love the non-Christian as a potential Christian does not mean that all our focus is on making the non-Christian into a Christian. After all, the parent's love for the child should typically be focused on the child as a potential adult. At the same time, much of the expression of that love is not focused on making the child into an adult. We feed, clothe and play with the child. Obviously, if we fail to feed the child, she may well die and fail to develop into an adult. But that is typically not what we are thinking: we are fulfilling the child's imminent need. And playing helps form the child, too, but again that is often not what one is thinking—instead, one may well simply be enjoying the game. Nonetheless, the expression of the love as a whole is shaped by the fact that the love is a love of the child as a potential adult.
Nonetheless, that we love the non-Christian as a potential Christian does mean that evangelization is a central aspect of our love for neighbor. This evangelization may or may not be in words, of course, and need not be conscious. The evangelization is an expression of the desire for union with the neighbor: a union as fellow members of Christ's body.
If this is right, then 1 John is describing the normative case where the potentiality for being a fellow member of the body of Christ has been fulfilled, just as when ethicists discuss relationships with others, they often talk of the case of adults, which is in some way the normative case. But the love for the child is in continuity with and directed towards becoming the love for the adult, and the love for a non-Christian is in continuity with and directed towards becoming the love for a Christian.
Is the comparison of the non-Christian to a child offensive? Well, it does fail to capture one aspect of the situation: the Christian's own falling short of being what she is called to. Perhaps a better way, in many cases, is to think of the case of a somewhat older sibling, or maybe of fifteen-year old parents, who in many ways are still children themselves.
