Shaking hands with a Christian or a Jew: Does it invalidate the ablution?

Q: If a man performs ablution for prayer, and on his way he encounters a Christian or a Jew and shakes hands with him, does his ablution become invalid? And what is the ruling on inviting a Christian to eat in a Muslim's house?

A: If a Muslim shakes hands with a Christian, a Jew, or any other disbeliever, the ablution is not invalidated by that. However, he is not permitted to shake hands with them, nor is he permitted to initiate the greeting with them.
This is based on the saying of the Prophet (peace and blessings be upon him): Shaking hands is more severe than initiating the greeting. Therefore, he should not initiate the greeting with them, nor shake hands with them, unless they initiate the greeting with him; in that case, he should shake hands with them. There is no harm in reciprocating, because he did not initiate it; rather, they were the ones who initiated it.
As for inviting them to a wedding feast and eating with them, this requires detailed explanation:
If he invites them in order to encourage them to embrace Islam, advise them, and guide them to Islam, then there is no harm in that. Likewise, if they are guests. However, he should not invite them to eat for the sake of friendship and companionship; he should not do that, because there is enmity and hatred between us and them, as Allah the Exalted said: [Surah al-Mumtahanah: 4]. And success is from Allah.