Revelation 2:4 - What is first love?

Rev 2:4  Nevertheless I have somewhat against thee, because thou hast left thy first love.

Gill writes about this here....

because thou hast left thy first love: by which is meant, not hospitality to strangers, or an affectionate care of the poor of the church, or a zealous concern to feed the flock, and maintain church discipline; but the love of the saints to God, and Christ, and one another, which appeared at the beginning of this church state, when they were all of one heart and one soul, as generally at first conversion love is the warmest; and so it was at the first planting of Gospel churches, and therefore here called first love. Now this, though it was not lost, for the true grace of love can never be lost, yet it was left; it abated in its heat and fervour; there was a remissness in the exercise of it; what our Lord had foretold should be before the destruction of Jerusalem was fulfilled in this period of time, the love of many waxed cold, Mat 24:12; through the prevalence of corruption in some; and through an over love to the world, as in Demas, and others; and through a desire of ease and freedom from reproach and persecution; and through the introduction of errors, which damp the heat of love, and spirit of religion; and through the contentions and divisions among themselves, as at Corinth, Galatia, and elsewhere, which greatly weakened their love to one another, and to divine things; and which was very displeasing to Christ, who, for the restoring of them, gives the following advice. Compare with this 2Ti 1:15.

Mat 24:12  And because iniquity shall abound, the love of many shall wax cold.

2 Timothy 1:15

15  This thou knowest, that all they which are in Asia be turned away from me; of whom are Phygellus and Hermogenes.

be turned away from me; were ashamed of him, because of his chain, and despised him under his afflictions, and had him in abhorrence and contempt, and revolted from his doctrine; though the defection was very general, and the apostle appeals to Timothy for the truth of it, as a fact well known to him: "this thou knowest"; Timothy being at Ephesus, which was in Asia; and since there was so great an apostasy in the country where he was, the above exhortations were very seasonable, to hold fast the form of sound words, and keep the good thing committed to him; seeing so many were falling off from the truth of the Gospel: