Haply - Perhaps.
To mock him - To ridicule him. To laugh at him.

Lest haply after he hath laid the foundation,.... Has begun to build, has taken up a profession, has submitted to ordinances, and got into a church state:
and is not able to finish it; a foundation may be laid, and the building may never be finished, because the foundation is not laid right; was it, it would continue, and the building go on, and at last be finished; though no man is able to finish it of himself, yet those hands which have laid the foundation, will raise up the superstructure, and complete the whole building, through the power and efficacy of divine grace: but where there is a beginning, and which at first looks well, and there is no progress, but the work is dropped and left unfinished,
all that behold it, begin to mock; as follows;